diff --git a/.traefik.yml b/.traefik.yml
index ddb18cc..091ff6b 100644
--- a/.traefik.yml
+++ b/.traefik.yml
@@ -77,6 +77,7 @@ testData:
   # Custom claim names for Auth0 and other providers with namespaced claims
   roleClaimName: roles # JWT claim name for extracting user roles (default: "roles")
   groupClaimName: groups # JWT claim name for extracting user groups (default: "groups")
+  userIdentifierClaim: email # JWT claim for user identification (default: "email", alternatives: "sub", "oid", "upn", "preferred_username")
 
   # ⚠️ CRITICAL for TLS termination scenarios (AWS ALB, Cloud Load Balancers, etc.)
   # When NOT specified in config: defaults to FALSE (Go zero value)
@@ -290,6 +291,26 @@ testDataWithRedis:
 #     - "AppRoleName"
 #   # See README.md "Provider Configuration Recommendations" for Azure AD.
 
+# --- Azure AD Users Without Email Example (Issue #95) ---
+# testDataAzureADNoEmail:
+#   providerURL: https://login.microsoftonline.com/your-tenant-id/v2.0
+#   clientID: your-azure-ad-client-id
+#   clientSecret: your-azure-ad-client-secret
+#   callbackURL: /oauth2/callback
+#   sessionEncryptionKey: "a-very-secure-key-at-least-32-bytes-long-for-azure"
+#   # Use 'sub' claim instead of 'email' for user identification
+#   userIdentifierClaim: sub  # or "oid", "upn", "preferred_username"
+#   overrideScopes: true  # Remove email scope if not needed
+#   scopes:
+#     - openid
+#     - profile
+#     - groups  # For group-based access control
+#   # When using non-email identifiers, allowedUsers matches against the claim value
+#   allowedUsers:
+#     - "abc12345-6789-0abc-def0-123456789abc"  # Azure AD user object ID (sub or oid claim)
+#   # NOTE: allowedUserDomains is ignored when userIdentifierClaim is not "email"
+#   # See: https://github.com/lukaszraczylo/traefikoidc/issues/95
+
 # --- Google Workspace / Google Cloud Identity Example ---
 # testDataGoogle:
 #   providerURL: https://accounts.google.com # Standard Google OIDC endpoint
@@ -608,6 +629,38 @@ configuration:
     items:
       type: string
 
+  userIdentifierClaim:
+    type: string
+    description: |
+      Specifies the JWT claim to use as the user identifier for authentication and authorization.
+
+      This allows authentication for users without email addresses, such as Azure AD service
+      accounts or organizational accounts that don't have email attributes configured.
+
+      When set to a non-email claim (e.g., "sub", "oid", "upn"):
+      - AllowedUsers will match against this claim value instead of email
+      - AllowedUserDomains validation is skipped (domains only apply to email addresses)
+      - The session stores this identifier as the user's identity
+      - If the configured claim is missing, falls back to "sub" (required by OIDC spec)
+
+      Common values by provider:
+      - Default: "email" (standard email-based identification)
+      - Azure AD: "sub", "oid" (object ID), "upn" (User Principal Name), "preferred_username"
+      - Generic OIDC: "sub" (always present per OIDC specification)
+      - Keycloak: "sub", "preferred_username"
+
+      Example for Azure AD users without email:
+      ```yaml
+      userIdentifierClaim: sub
+      allowedUsers:
+        - "abc123-user-object-id"
+        - "xyz789-another-user-id"
+      ```
+
+      Default: "email"
+      See: https://github.com/lukaszraczylo/traefikoidc/issues/95
+    required: false
+
   revocationURL:
     type: string
     description: |
diff --git a/README.md b/README.md
index e369dc6..82ac10f 100644
--- a/README.md
+++ b/README.md
@@ -124,6 +124,7 @@ The middleware supports the following configuration options:
 | `allowedRolesAndGroups` | Restricts access to users with specific roles or groups | none | `["admin", "developer"]` |
 | `roleClaimName` | JWT claim name for extracting user roles (supports namespaced claims for Auth0) | `"roles"` | `"https://myapp.com/roles"`, `"user_roles"` |
 | `groupClaimName` | JWT claim name for extracting user groups (supports namespaced claims for Auth0) | `"groups"` | `"https://myapp.com/groups"`, `"user_groups"` |
+| `userIdentifierClaim` | JWT claim to use as user identifier (for users without email, e.g., Azure AD service accounts) | `"email"` | `"sub"`, `"oid"`, `"upn"`, `"preferred_username"` |
 | `revocationURL` | The endpoint for revoking tokens | auto-discovered | `https://accounts.google.com/revoke` |
 | `oidcEndSessionURL` | The provider's end session endpoint | auto-discovered | `https://accounts.google.com/logout` |
 | `enablePKCE` | Enables PKCE (Proof Key for Code Exchange) for authorization code flow | `false` | `true`, `false` |
@@ -1242,6 +1243,45 @@ spec:
         - "AppRoleName"        # Application role names
 ```
 
+### Azure AD Configuration (Users Without Email)
+
+For Azure AD users without email addresses (service accounts, organizational accounts without mail attributes):
+
+```yaml
+apiVersion: traefik.io/v1alpha1
+kind: Middleware
+metadata:
+  name: oidc-azure-no-email
+  namespace: traefik
+spec:
+  plugin:
+    traefikoidc:
+      providerURL: https://login.microsoftonline.com/your-tenant-id/v2.0
+      clientID: your-azure-ad-client-id
+      clientSecret: your-azure-ad-client-secret
+      sessionEncryptionKey: your-secure-encryption-key-min-32-chars
+      callbackURL: /oauth2/callback
+      logoutURL: /oauth2/logout
+
+      # Use 'sub' instead of 'email' for user identification
+      userIdentifierClaim: sub  # Can also use: "oid", "upn", "preferred_username"
+
+      overrideScopes: true  # Optional: Don't request email scope if not needed
+      scopes:
+        - openid
+        - profile
+        - groups
+
+      # When using non-email identifiers, allowedUsers matches against the claim value
+      allowedUsers:
+        - "abc12345-6789-0abc-def0-123456789abc"  # Azure AD user object ID
+        - "def67890-1234-5678-90ab-cdef12345678"
+
+      # NOTE: allowedUserDomains is ignored when userIdentifierClaim is not "email"
+```
+
+> **Note**: When `userIdentifierClaim` is set to a non-email claim (like `sub`, `oid`, or `upn`), the `allowedUserDomains` configuration is ignored since domain-based validation only applies to email addresses. Use `allowedUsers` with the actual claim values instead.
+
 ### Auth0 Configuration
 
 ```yaml
diff --git a/audience_validation_test.go b/audience_validation_test.go
index 8e07184..7fe7aa7 100644
--- a/audience_validation_test.go
+++ b/audience_validation_test.go
@@ -849,26 +849,27 @@ func TestAudienceEndToEndScenario(t *testing.T) {
 	customAudience := "https://api.company.com"
 
 	tOidc := &TraefikOidc{
-		next:               nextHandler,
-		name:               "test",
-		redirURLPath:       "/callback",
-		logoutURLPath:      "/callback/logout",
-		issuerURL:          "https://auth.company.com",
-		clientID:           "test-client-id",
-		clientSecret:       "test-client-secret",
-		audience:           customAudience, // Set custom audience
-		jwkCache:           mockJWKCache,
-		jwksURL:            "https://auth.company.com/.well-known/jwks.json",
-		tokenBlacklist:     tokenBlacklist,
-		tokenCache:         tokenCache,
-		limiter:            rate.NewLimiter(rate.Every(time.Second), 10),
-		logger:             logger,
-		allowedUserDomains: map[string]struct{}{"company.com": {}},
-		excludedURLs:       map[string]struct{}{},
-		httpClient:         &http.Client{},
-		initComplete:       make(chan struct{}),
-		sessionManager:     sm,
-		extractClaimsFunc:  extractClaims,
+		next:                nextHandler,
+		name:                "test",
+		redirURLPath:        "/callback",
+		logoutURLPath:       "/callback/logout",
+		issuerURL:           "https://auth.company.com",
+		clientID:            "test-client-id",
+		clientSecret:        "test-client-secret",
+		audience:            customAudience, // Set custom audience
+		jwkCache:            mockJWKCache,
+		jwksURL:             "https://auth.company.com/.well-known/jwks.json",
+		tokenBlacklist:      tokenBlacklist,
+		tokenCache:          tokenCache,
+		limiter:             rate.NewLimiter(rate.Every(time.Second), 10),
+		logger:              logger,
+		allowedUserDomains:  map[string]struct{}{"company.com": {}},
+		userIdentifierClaim: "email", // Required for user identification
+		excludedURLs:        map[string]struct{}{},
+		httpClient:          &http.Client{},
+		initComplete:        make(chan struct{}),
+		sessionManager:      sm,
+		extractClaimsFunc:   extractClaims,
 	}
 	tOidc.jwtVerifier = tOidc
 	tOidc.tokenVerifier = tOidc
diff --git a/auth_flow.go b/auth_flow.go
index 5b0cb9f..505c532 100644
--- a/auth_flow.go
+++ b/auth_flow.go
@@ -223,15 +223,25 @@ func (t *TraefikOidc) handleCallback(rw http.ResponseWriter, req *http.Request,
 		return
 	}
 
-	email, _ := claims["email"].(string)
-	if email == "" {
-		t.logger.Errorf("Email claim missing or empty in token during callback")
-		t.sendErrorResponse(rw, req, "Authentication failed: Email missing in token", http.StatusInternalServerError)
-		return
+	// Extract user identifier from the configured claim (defaults to "email" for backward compatibility)
+	userIdentifier, _ := claims[t.userIdentifierClaim].(string)
+	if userIdentifier == "" {
+		// Try "sub" as fallback since it's required by OIDC spec
+		if t.userIdentifierClaim != "sub" {
+			userIdentifier, _ = claims["sub"].(string)
+		}
+		if userIdentifier == "" {
+			t.logger.Errorf("User identifier claim '%s' missing or empty in token during callback", t.userIdentifierClaim)
+			t.sendErrorResponse(rw, req, "Authentication failed: User identifier missing in token", http.StatusInternalServerError)
+			return
+		}
+		t.logger.Debugf("Configured claim '%s' not found, using 'sub' claim as fallback", t.userIdentifierClaim)
 	}
-	if !t.isAllowedDomain(email) {
-		t.logger.Errorf("Disallowed email domain during callback: %s", email)
-		t.sendErrorResponse(rw, req, "Authentication failed: Email domain not allowed", http.StatusForbidden)
+
+	// Validate user authorization
+	if !t.isAllowedUser(userIdentifier) {
+		t.logger.Errorf("User not authorized during callback: %s", userIdentifier)
+		t.sendErrorResponse(rw, req, "Authentication failed: User not authorized", http.StatusForbidden)
 		return
 	}
 
@@ -240,7 +250,7 @@ func (t *TraefikOidc) handleCallback(rw http.ResponseWriter, req *http.Request,
 		t.sendErrorResponse(rw, req, "Failed to update session", http.StatusInternalServerError)
 		return
 	}
-	session.SetEmail(email)
+	session.SetEmail(userIdentifier) // SetEmail stores the user identifier (email or other claim)
 	session.SetIDToken(tokenResponse.IDToken)
 	session.SetAccessToken(tokenResponse.AccessToken)
 	session.SetRefreshToken(tokenResponse.RefreshToken)
diff --git a/handlers/oauth_handler.go b/handlers/oauth_handler.go
index 055d4f6..2a1f1d3 100644
--- a/handlers/oauth_handler.go
+++ b/handlers/oauth_handler.go
@@ -15,7 +15,8 @@ type OAuthHandler struct {
 	tokenExchanger        TokenExchanger
 	tokenVerifier         TokenVerifier
 	extractClaimsFunc     func(tokenString string) (map[string]interface{}, error)
-	isAllowedDomainFunc   func(email string) bool
+	isAllowedUserFunc     func(userIdentifier string) bool // validates user authorization
+	userIdentifierClaim   string                           // JWT claim to use for user identification
 	redirURLPath          string
 	sendErrorResponseFunc func(rw http.ResponseWriter, req *http.Request, message string, code int)
 }
@@ -77,16 +78,22 @@ type TokenResponse struct {
 // NewOAuthHandler creates a new OAuth handler
 func NewOAuthHandler(logger Logger, sessionManager SessionManager, tokenExchanger TokenExchanger,
 	tokenVerifier TokenVerifier, extractClaimsFunc func(string) (map[string]interface{}, error),
-	isAllowedDomainFunc func(string) bool, redirURLPath string,
+	isAllowedUserFunc func(string) bool, userIdentifierClaim string, redirURLPath string,
 	sendErrorResponseFunc func(http.ResponseWriter, *http.Request, string, int)) *OAuthHandler {
 
+	// Default to "email" for backward compatibility
+	if userIdentifierClaim == "" {
+		userIdentifierClaim = "email"
+	}
+
 	return &OAuthHandler{
 		logger:                logger,
 		sessionManager:        sessionManager,
 		tokenExchanger:        tokenExchanger,
 		tokenVerifier:         tokenVerifier,
 		extractClaimsFunc:     extractClaimsFunc,
-		isAllowedDomainFunc:   isAllowedDomainFunc,
+		isAllowedUserFunc:     isAllowedUserFunc,
+		userIdentifierClaim:   userIdentifierClaim,
 		redirURLPath:          redirURLPath,
 		sendErrorResponseFunc: sendErrorResponseFunc,
 	}
@@ -225,15 +232,25 @@ func (h *OAuthHandler) HandleCallback(rw http.ResponseWriter, req *http.Request,
 		return
 	}
 
-	email, _ := claims["email"].(string)
-	if email == "" {
-		h.logger.Errorf("Email claim missing or empty in token during callback")
-		h.sendErrorResponseFunc(rw, req, "Authentication failed: Email missing in token", http.StatusInternalServerError)
-		return
+	// Extract user identifier from the configured claim (defaults to "email" for backward compatibility)
+	userIdentifier, _ := claims[h.userIdentifierClaim].(string)
+	if userIdentifier == "" {
+		// Try "sub" as fallback since it's required by OIDC spec
+		if h.userIdentifierClaim != "sub" {
+			userIdentifier, _ = claims["sub"].(string)
+		}
+		if userIdentifier == "" {
+			h.logger.Errorf("User identifier claim '%s' missing or empty in token during callback", h.userIdentifierClaim)
+			h.sendErrorResponseFunc(rw, req, "Authentication failed: User identifier missing in token", http.StatusInternalServerError)
+			return
+		}
+		h.logger.Debugf("Configured claim '%s' not found, using 'sub' claim as fallback", h.userIdentifierClaim)
 	}
-	if !h.isAllowedDomainFunc(email) {
-		h.logger.Errorf("Disallowed email domain during callback: %s", email)
-		h.sendErrorResponseFunc(rw, req, "Authentication failed: Email domain not allowed", http.StatusForbidden)
+
+	// Validate user authorization
+	if !h.isAllowedUserFunc(userIdentifier) {
+		h.logger.Errorf("User not authorized during callback: %s", userIdentifier)
+		h.sendErrorResponseFunc(rw, req, "Authentication failed: User not authorized", http.StatusForbidden)
 		return
 	}
 
@@ -242,7 +259,7 @@ func (h *OAuthHandler) HandleCallback(rw http.ResponseWriter, req *http.Request,
 		h.sendErrorResponseFunc(rw, req, "Failed to update session", http.StatusInternalServerError)
 		return
 	}
-	session.SetEmail(email)
+	session.SetEmail(userIdentifier) // SetEmail stores the user identifier (email or other claim)
 	session.SetIDToken(tokenResponse.IDToken)
 	session.SetAccessToken(tokenResponse.AccessToken)
 	session.SetRefreshToken(tokenResponse.RefreshToken)
diff --git a/handlers/oauth_handler_test.go b/handlers/oauth_handler_test.go
index 2e3c9f0..615ce55 100644
--- a/handlers/oauth_handler_test.go
+++ b/handlers/oauth_handler_test.go
@@ -108,11 +108,11 @@ func TestOAuthHandler_NewOAuthHandler(t *testing.T) {
 		return map[string]interface{}{"email": "test@example.com", "nonce": "test-nonce"}, nil
 	}
 
-	isAllowed := func(email string) bool { return true }
+	isAllowedUser := func(userIdentifier string) bool { return true }
 	sendError := func(rw http.ResponseWriter, req *http.Request, msg string, code int) {}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowedUser, "email", "/callback", sendError)
 
 	if handler == nil {
 		t.Fatal("Expected handler to be created, got nil")
@@ -151,7 +151,7 @@ func TestOAuthHandler_HandleCallback_SessionError(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test&state=test", nil)
 	rw := httptest.NewRecorder()
@@ -190,7 +190,7 @@ func TestOAuthHandler_HandleCallback_ProviderError(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	// Test with error parameter
 	req := httptest.NewRequest("GET", "/callback?error=access_denied&error_description=User%20denied%20access", nil)
@@ -230,7 +230,7 @@ func TestOAuthHandler_HandleCallback_MissingState(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test", nil)
 	rw := httptest.NewRecorder()
@@ -265,7 +265,7 @@ func TestOAuthHandler_HandleCallback_MissingCSRF(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -300,7 +300,7 @@ func TestOAuthHandler_HandleCallback_CSRFMismatch(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -335,7 +335,7 @@ func TestOAuthHandler_HandleCallback_MissingCode(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -370,7 +370,7 @@ func TestOAuthHandler_HandleCallback_TokenExchangeError(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test-code&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -406,7 +406,7 @@ func TestOAuthHandler_HandleCallback_TokenVerificationError(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test-code&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -444,7 +444,7 @@ func TestOAuthHandler_HandleCallback_ClaimsExtractionError(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test-code&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -483,7 +483,7 @@ func TestOAuthHandler_HandleCallback_MissingNonceInToken(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test-code&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -521,7 +521,7 @@ func TestOAuthHandler_HandleCallback_MissingNonceInSession(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test-code&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -559,7 +559,7 @@ func TestOAuthHandler_HandleCallback_NonceMismatch(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test-code&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -591,13 +591,13 @@ func TestOAuthHandler_HandleCallback_MissingEmail(t *testing.T) {
 		if code != http.StatusInternalServerError {
 			t.Errorf("Expected status %d, got %d", http.StatusInternalServerError, code)
 		}
-		if !strings.Contains(msg, "Email missing in token") {
-			t.Errorf("Expected error message to contain 'Email missing in token', got '%s'", msg)
+		if !strings.Contains(msg, "User identifier missing in token") {
+			t.Errorf("Expected error message to contain 'User identifier missing in token', got '%s'", msg)
 		}
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test-code&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -629,13 +629,13 @@ func TestOAuthHandler_HandleCallback_DisallowedDomain(t *testing.T) {
 		if code != http.StatusForbidden {
 			t.Errorf("Expected status %d, got %d", http.StatusForbidden, code)
 		}
-		if !strings.Contains(msg, "Email domain not allowed") {
-			t.Errorf("Expected error message to contain 'Email domain not allowed', got '%s'", msg)
+		if !strings.Contains(msg, "User not authorized") {
+			t.Errorf("Expected error message to contain 'User not authorized', got '%s'", msg)
 		}
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test-code&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -677,7 +677,7 @@ func TestOAuthHandler_HandleCallback_SessionSaveError(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test-code&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -719,7 +719,7 @@ func TestOAuthHandler_HandleCallback_SetAuthenticatedError(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test-code&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -760,7 +760,7 @@ func TestOAuthHandler_HandleCallback_Success(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test-code&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -843,7 +843,7 @@ func TestOAuthHandler_HandleCallback_SuccessDefaultRedirect(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test-code&state=test-state", nil)
 	rw := httptest.NewRecorder()
@@ -884,7 +884,7 @@ func TestOAuthHandler_HandleCallback_RedirectURLPathExcluded(t *testing.T) {
 	}
 
 	handler := NewOAuthHandler(logger, sessionManager, tokenExchanger, tokenVerifier,
-		extractClaims, isAllowed, "/callback", sendError)
+		extractClaims, isAllowed, "email", "/callback", sendError)
 
 	req := httptest.NewRequest("GET", "/callback?code=test-code&state=test-state", nil)
 	rw := httptest.NewRecorder()
diff --git a/main.go b/main.go
index a338774..1ebbd8e 100644
--- a/main.go
+++ b/main.go
@@ -177,6 +177,12 @@ func NewWithContext(ctx context.Context, config *Config, next http.Handler, name
 			}
 			return "groups" // Backward compatible default
 		}(),
+		userIdentifierClaim: func() string {
+			if config.UserIdentifierClaim != "" {
+				return config.UserIdentifierClaim
+			}
+			return "email" // Backward compatible default
+		}(),
 		forceHTTPS:                config.ForceHTTPS,
 		enablePKCE:                config.EnablePKCE,
 		overrideScopes:            config.OverrideScopes,
diff --git a/main_test.go b/main_test.go
index 3da5b2e..57a2d9e 100644
--- a/main_test.go
+++ b/main_test.go
@@ -122,22 +122,23 @@ func (ts *TestSuite) Setup() {
 
 	// Common TraefikOidc instance
 	ts.tOidc = &TraefikOidc{
-		issuerURL:          "https://test-issuer.com",
-		clientID:           "test-client-id",
-		audience:           "test-client-id",
-		clientSecret:       "test-client-secret",
-		roleClaimName:      "roles",  // Set default for backward compatibility
-		groupClaimName:     "groups", // Set default for backward compatibility
-		jwkCache:           ts.mockJWKCache,
-		jwksURL:            "https://test-jwks-url.com",
-		revocationURL:      "https://revocation-endpoint.com",
-		limiter:            rate.NewLimiter(rate.Every(time.Second), 10),
-		tokenBlacklist:     tokenBlacklist,
-		tokenCache:         tokenCache,
-		logger:             logger,
-		allowedUserDomains: map[string]struct{}{"example.com": {}},
-		excludedURLs:       map[string]struct{}{"/favicon": {}, "/health": {}},
-		httpClient:         &http.Client{Timeout: 10 * time.Second},
+		issuerURL:           "https://test-issuer.com",
+		clientID:            "test-client-id",
+		audience:            "test-client-id",
+		clientSecret:        "test-client-secret",
+		roleClaimName:       "roles",  // Set default for backward compatibility
+		groupClaimName:      "groups", // Set default for backward compatibility
+		userIdentifierClaim: "email",  // Set default for backward compatibility
+		jwkCache:            ts.mockJWKCache,
+		jwksURL:             "https://test-jwks-url.com",
+		revocationURL:       "https://revocation-endpoint.com",
+		limiter:             rate.NewLimiter(rate.Every(time.Second), 10),
+		tokenBlacklist:      tokenBlacklist,
+		tokenCache:          tokenCache,
+		logger:              logger,
+		allowedUserDomains:  map[string]struct{}{"example.com": {}},
+		excludedURLs:        map[string]struct{}{"/favicon": {}, "/health": {}},
+		httpClient:          &http.Client{Timeout: 10 * time.Second},
 		// Explicitly set paths as New() is bypassed
 		redirURLPath:            "/callback",                     // Assume default callback path for tests
 		logoutURLPath:           "/callback/logout",              // Assume default logout path for tests
@@ -784,7 +785,7 @@ func TestServeHTTP(t *testing.T) {
 				"Accept": "application/json",
 			},
 			expectedStatus: http.StatusForbidden,
-			expectedBody:   `{"error":"Forbidden","error_description":"Access denied: Your email domain is not allowed. To log out, visit: /callback/logout","status_code":403}`,
+			expectedBody:   `{"error":"Forbidden","error_description":"Access denied: You are not authorized to access this resource. To log out, visit: /callback/logout","status_code":403}`,
 		},
 		{
 			name:        "Disallowed Domain (Accept: HTML)",
@@ -1282,8 +1283,9 @@ func TestHandleCallback(t *testing.T) {
 				instanceExtractClaimsFunc = extractClaims // Default to the real function if not provided by test case
 			}
 			tOidc := &TraefikOidc{
-				allowedUserDomains: map[string]struct{}{"example.com": {}},
-				logger:             logger,
+				allowedUserDomains:  map[string]struct{}{"example.com": {}},
+				logger:              logger,
+				userIdentifierClaim: "email", // Required for claim extraction
 				// exchangeCodeForTokenFunc: tc.exchangeCodeForToken, // Removed field
 				extractClaimsFunc: instanceExtractClaimsFunc, // Use the potentially defaulted function
 				tokenVerifier:     nil,                       // Will be set to self below
@@ -1438,6 +1440,228 @@ func TestIsAllowedDomain(t *testing.T) {
 	}
 }
 
+func TestIsAllowedUser(t *testing.T) {
+	ts := NewTestSuite(t)
+	ts.Setup()
+
+	tests := []struct {
+		allowedDomains      map[string]struct{}
+		allowedUsers        map[string]struct{}
+		userIdentifierClaim string
+		name                string
+		userIdentifier      string
+		allowed             bool
+	}{
+		// Email-based identification (default behavior)
+		{
+			name:                "Email identifier - allowed domain",
+			userIdentifier:      "user@example.com",
+			userIdentifierClaim: "email",
+			allowedDomains:      map[string]struct{}{"example.com": {}},
+			allowedUsers:        map[string]struct{}{},
+			allowed:             true,
+		},
+		{
+			name:                "Email identifier - disallowed domain",
+			userIdentifier:      "user@notallowed.com",
+			userIdentifierClaim: "email",
+			allowedDomains:      map[string]struct{}{"example.com": {}},
+			allowedUsers:        map[string]struct{}{},
+			allowed:             false,
+		},
+		{
+			name:                "Email identifier - specific user allowed",
+			userIdentifier:      "specific.user@otherdomain.com",
+			userIdentifierClaim: "email",
+			allowedDomains:      map[string]struct{}{"example.com": {}},
+			allowedUsers:        map[string]struct{}{"specific.user@otherdomain.com": {}},
+			allowed:             true,
+		},
+
+		// Non-email identifier (sub claim - for Azure AD users without email)
+		{
+			name:                "Sub identifier - allowed in allowedUsers",
+			userIdentifier:      "abc12345-6789-0abc-def0-123456789abc",
+			userIdentifierClaim: "sub",
+			allowedDomains:      map[string]struct{}{},
+			allowedUsers:        map[string]struct{}{"abc12345-6789-0abc-def0-123456789abc": {}},
+			allowed:             true,
+		},
+		{
+			name:                "Sub identifier - not in allowedUsers",
+			userIdentifier:      "xyz-not-allowed-user",
+			userIdentifierClaim: "sub",
+			allowedDomains:      map[string]struct{}{},
+			allowedUsers:        map[string]struct{}{"abc12345-6789-0abc-def0-123456789abc": {}},
+			allowed:             false,
+		},
+		{
+			name:                "Sub identifier - allowedDomains ignored for non-email",
+			userIdentifier:      "user-id-12345",
+			userIdentifierClaim: "sub",
+			allowedDomains:      map[string]struct{}{"example.com": {}}, // Should be ignored
+			allowedUsers:        map[string]struct{}{"user-id-12345": {}},
+			allowed:             true,
+		},
+		{
+			name:                "Sub identifier - no restrictions allows all",
+			userIdentifier:      "any-user-id",
+			userIdentifierClaim: "sub",
+			allowedDomains:      map[string]struct{}{},
+			allowedUsers:        map[string]struct{}{},
+			allowed:             true,
+		},
+		{
+			name:                "Sub identifier - case insensitive matching",
+			userIdentifier:      "ABC12345-6789-0ABC-DEF0-123456789ABC", // Uppercase
+			userIdentifierClaim: "sub",
+			allowedDomains:      map[string]struct{}{},
+			allowedUsers:        map[string]struct{}{"abc12345-6789-0abc-def0-123456789abc": {}}, // Lowercase
+			allowed:             true,
+		},
+
+		// OID claim (Azure AD object ID)
+		{
+			name:                "OID identifier - allowed user",
+			userIdentifier:      "oid-12345-67890",
+			userIdentifierClaim: "oid",
+			allowedDomains:      map[string]struct{}{},
+			allowedUsers:        map[string]struct{}{"oid-12345-67890": {}},
+			allowed:             true,
+		},
+
+		// UPN claim (Azure AD User Principal Name)
+		{
+			name:                "UPN identifier - allowed user (looks like email but use sub logic)",
+			userIdentifier:      "user@tenant.onmicrosoft.com",
+			userIdentifierClaim: "upn",
+			allowedDomains:      map[string]struct{}{"example.com": {}}, // Different domain, should be ignored
+			allowedUsers:        map[string]struct{}{"user@tenant.onmicrosoft.com": {}},
+			allowed:             true,
+		},
+
+		// Edge cases
+		{
+			name:                "Empty identifier - not allowed",
+			userIdentifier:      "",
+			userIdentifierClaim: "sub",
+			allowedDomains:      map[string]struct{}{},
+			allowedUsers:        map[string]struct{}{"some-user": {}},
+			allowed:             false,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			// Configure TraefikOidc instance for this test case
+			tOidc := ts.tOidc
+			tOidc.allowedUserDomains = tc.allowedDomains
+			tOidc.allowedUsers = tc.allowedUsers
+			tOidc.userIdentifierClaim = tc.userIdentifierClaim
+
+			allowed := tOidc.isAllowedUser(tc.userIdentifier)
+			if allowed != tc.allowed {
+				t.Errorf("Expected allowed=%v, got %v for userIdentifier=%q with claim=%q",
+					tc.allowed, allowed, tc.userIdentifier, tc.userIdentifierClaim)
+			}
+		})
+	}
+}
+
+func TestUserIdentifierClaimExtraction(t *testing.T) {
+	// Test that the correct claim is extracted based on userIdentifierClaim config
+	tests := []struct {
+		name                string
+		userIdentifierClaim string
+		claims              map[string]interface{}
+		expectedIdentifier  string
+		shouldFallbackToSub bool
+	}{
+		{
+			name:                "Email claim extraction (default)",
+			userIdentifierClaim: "email",
+			claims: map[string]interface{}{
+				"sub":   "user-sub-id",
+				"email": "user@example.com",
+			},
+			expectedIdentifier:  "user@example.com",
+			shouldFallbackToSub: false,
+		},
+		{
+			name:                "Sub claim extraction",
+			userIdentifierClaim: "sub",
+			claims: map[string]interface{}{
+				"sub":   "user-sub-id",
+				"email": "user@example.com",
+			},
+			expectedIdentifier:  "user-sub-id",
+			shouldFallbackToSub: false,
+		},
+		{
+			name:                "OID claim extraction (Azure AD)",
+			userIdentifierClaim: "oid",
+			claims: map[string]interface{}{
+				"sub":   "user-sub-id",
+				"email": "user@example.com",
+				"oid":   "azure-object-id",
+			},
+			expectedIdentifier:  "azure-object-id",
+			shouldFallbackToSub: false,
+		},
+		{
+			name:                "UPN claim extraction (Azure AD)",
+			userIdentifierClaim: "upn",
+			claims: map[string]interface{}{
+				"sub": "user-sub-id",
+				"upn": "user@tenant.onmicrosoft.com",
+			},
+			expectedIdentifier:  "user@tenant.onmicrosoft.com",
+			shouldFallbackToSub: false,
+		},
+		{
+			name:                "Fallback to sub when configured claim is missing",
+			userIdentifierClaim: "email",
+			claims: map[string]interface{}{
+				"sub": "fallback-sub-id",
+				// email is missing
+			},
+			expectedIdentifier:  "fallback-sub-id",
+			shouldFallbackToSub: true,
+		},
+		{
+			name:                "preferred_username claim extraction",
+			userIdentifierClaim: "preferred_username",
+			claims: map[string]interface{}{
+				"sub":                "user-sub-id",
+				"preferred_username": "jdoe",
+			},
+			expectedIdentifier:  "jdoe",
+			shouldFallbackToSub: false,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			// Extract user identifier using the same logic as auth_flow.go
+			userIdentifier, _ := tc.claims[tc.userIdentifierClaim].(string)
+			usedFallback := false
+
+			if userIdentifier == "" && tc.userIdentifierClaim != "sub" {
+				userIdentifier, _ = tc.claims["sub"].(string)
+				usedFallback = true
+			}
+
+			if userIdentifier != tc.expectedIdentifier {
+				t.Errorf("Expected identifier %q, got %q", tc.expectedIdentifier, userIdentifier)
+			}
+
+			if usedFallback != tc.shouldFallbackToSub {
+				t.Errorf("Expected fallback=%v, got %v", tc.shouldFallbackToSub, usedFallback)
+			}
+		})
+	}
+}
+
 func TestOIDCHandler(t *testing.T) {
 	ts := NewTestSuite(t)
 	ts.Setup()
diff --git a/middleware.go b/middleware.go
index 9d103e4..518ddd9 100644
--- a/middleware.go
+++ b/middleware.go
@@ -125,12 +125,12 @@ func (t *TraefikOidc) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		return
 	}
 
-	email := session.GetEmail()
-	// Domain restriction check removed debug output
-	if authenticated && email != "" {
-		if !t.isAllowedDomain(email) {
-			t.logger.Infof("User with email %s is not from an allowed domain", email)
-			errorMsg := fmt.Sprintf("Access denied: Your email domain is not allowed. To log out, visit: %s", t.logoutURLPath)
+	userIdentifier := session.GetEmail() // GetEmail returns the stored user identifier (email or other claim)
+	// User authorization check
+	if authenticated && userIdentifier != "" {
+		if !t.isAllowedUser(userIdentifier) {
+			t.logger.Infof("User %s is not authorized", userIdentifier)
+			errorMsg := fmt.Sprintf("Access denied: You are not authorized to access this resource. To log out, visit: %s", t.logoutURLPath)
 			t.sendErrorResponse(rw, req, errorMsg, http.StatusForbidden)
 			return
 		}
@@ -193,10 +193,10 @@ func (t *TraefikOidc) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 
 		refreshed := t.refreshToken(rw, req, session)
 		if refreshed {
-			email = session.GetEmail()
-			if email != "" && !t.isAllowedDomain(email) {
-				t.logger.Infof("User with refreshed token email %s is not from an allowed domain", email)
-				errorMsg := fmt.Sprintf("Access denied: Your email domain is not allowed. To log out, visit: %s", t.logoutURLPath)
+			userIdentifier = session.GetEmail() // GetEmail returns the stored user identifier
+			if userIdentifier != "" && !t.isAllowedUser(userIdentifier) {
+				t.logger.Infof("User with refreshed token %s is not authorized", userIdentifier)
+				errorMsg := fmt.Sprintf("Access denied: You are not authorized to access this resource. To log out, visit: %s", t.logoutURLPath)
 				t.sendErrorResponse(rw, req, errorMsg, http.StatusForbidden)
 				return
 			}
diff --git a/settings.go b/settings.go
index 9a2540d..19671b6 100644
--- a/settings.go
+++ b/settings.go
@@ -127,6 +127,22 @@ type Config struct {
 	// Default: "groups"
 	GroupClaimName string `json:"groupClaimName,omitempty"`
 
+	// UserIdentifierClaim specifies the JWT claim to use as the user identifier.
+	// This allows authentication for users without email addresses (e.g., Azure AD service accounts).
+	//
+	// Examples:
+	//   - Default (backward compatible): "email"
+	//   - Azure AD without email: "sub", "oid", "upn", or "preferred_username"
+	//   - Generic OIDC: "sub" (always present per OIDC spec)
+	//
+	// When set to a non-email claim:
+	//   - AllowedUsers will match against this claim value instead of email
+	//   - AllowedUserDomains validation is skipped (domains only apply to email)
+	//   - The session will store this identifier as the user's identity
+	//
+	// Default: "email"
+	UserIdentifierClaim string `json:"userIdentifierClaim,omitempty"`
+
 	// DynamicClientRegistration enables OIDC Dynamic Client Registration (RFC 7591)
 	// When enabled, the middleware will automatically register as a client with
 	// the OIDC provider if ClientID/ClientSecret are not provided.
diff --git a/types.go b/types.go
index ad66337..f4172dc 100644
--- a/types.go
+++ b/types.go
@@ -99,6 +99,7 @@ type TraefikOidc struct {
 	audience                   string // Expected JWT audience, defaults to clientID
 	roleClaimName              string // JWT claim name for extracting roles, defaults to "roles"
 	groupClaimName             string // JWT claim name for extracting groups, defaults to "groups"
+	userIdentifierClaim        string // JWT claim for user identification, defaults to "email"
 	name                       string
 	redirURLPath               string
 	logoutURLPath              string
diff --git a/utilities.go b/utilities.go
index ddfbc57..dce4518 100644
--- a/utilities.go
+++ b/utilities.go
@@ -55,6 +55,51 @@ func (t *TraefikOidc) safeLogInfo(msg string) {
 // DOMAIN VALIDATION
 // =============================================================================
 
+// isAllowedUser checks if a user identifier is authorized based on the configured user identifier claim.
+// When using email as the identifier (default), it validates against allowedUsers and allowedUserDomains.
+// When using non-email identifiers (sub, oid, upn, etc.), it only validates against allowedUsers
+// since domain-based validation doesn't apply to non-email identifiers.
+//
+// Parameters:
+//   - userIdentifier: The user identifier to validate (email, sub, oid, upn, etc.).
+//
+// Returns:
+//   - true if the user is authorized, false otherwise.
+func (t *TraefikOidc) isAllowedUser(userIdentifier string) bool {
+	// If no restrictions are configured, allow all authenticated users
+	if len(t.allowedUserDomains) == 0 && len(t.allowedUsers) == 0 {
+		return true
+	}
+
+	// Check if user is explicitly allowed
+	if len(t.allowedUsers) > 0 {
+		_, userAllowed := t.allowedUsers[strings.ToLower(userIdentifier)]
+		if userAllowed {
+			t.logger.Debugf("User identifier %s is explicitly allowed in allowedUsers", userIdentifier)
+			return true
+		}
+	}
+
+	// For email-based identifiers, also check domain restrictions
+	// Only apply domain validation if using email as identifier AND identifier looks like an email
+	if t.userIdentifierClaim == "email" && strings.Contains(userIdentifier, "@") {
+		return t.isAllowedDomain(userIdentifier)
+	}
+
+	// For non-email identifiers with allowedUserDomains configured, log a warning
+	if len(t.allowedUserDomains) > 0 && t.userIdentifierClaim != "email" {
+		t.logger.Debugf("AllowedUserDomains is configured but userIdentifierClaim is '%s', not 'email'. Domain validation skipped for: %s",
+			t.userIdentifierClaim, userIdentifier)
+	}
+
+	// User not found in allowedUsers list
+	if len(t.allowedUsers) > 0 {
+		t.logger.Debugf("User identifier %s is not in the allowed users list", userIdentifier)
+	}
+
+	return false
+}
+
 // isAllowedDomain checks if an email address is authorized based on domain or user whitelist.
 // It validates against both allowed user domains and specific allowed users.
 // Parameters: