diff --git a/CHANGELOG.md b/CHANGELOG.md
index a06c031..e6e316d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -27,6 +27,9 @@ All notable changes to this project are documented in this file. The format foll
 - "Link Microsoft account" on the profile screen (the signed-in user binds their own Microsoft account) and an administrator-assigned Microsoft account (UPN) per user – for administrators whose user principal name differs from their e-mail address. Sign-in finds bound accounts by object ID first, then by assigned UPN, then by e-mail.
 - Privacy exporter and eraser for the data the plugin copies.
 
+### Fixed
+- Accounts whose WordPress e-mail is the user principal name (while the Microsoft mail differs) were not found at sign-in and got a duplicate account from the sync; sign-in and sync now try the mail address and the UPN.
+
 ### Security
 Third audit (details: docs/security-audit.md, section 7): XML-RPC `system.multicall` bypass of button-only mode closed; privileged-account rules extended (bind_oid off, multisite-wide capabilities, code/HTML capabilities, deactivated administrators); cookie protection on WordPress 6.0/6.1; atomic and refreshed run lock with crash report; per-account tenant for deprovisioning; demotion safety stop; safety stop based on accounts linked before the run; photos re-encoded and removed on deactivation; one-time hardening of accounts deactivated by 1.0.
 
diff --git a/README.md b/README.md
index 0d8d55c..c816895 100644
--- a/README.md
+++ b/README.md
@@ -224,7 +224,7 @@ Alles wird live in der Vorschau angezeigt, bevor du speicherst.
 | Option | Standard | Beschreibung |
 | --- | --- | --- |
 | Konto an Microsoft-Objekt-ID binden | an | Beim ersten Login wird die `oid` gespeichert; danach muss sie übereinstimmen. Schützt vor Übernahme, wenn eine E-Mail-Adresse in Microsoft neu vergeben wird. |
-| UPN-Fallback | an | Fehlt der `email`-Claim, wird der User Principal Name verwendet, sofern er eine gültige E-Mail-Adresse ist. |
+| UPN-Fallback | an | Neben dem `email`-Claim wird auch der User Principal Name gesucht – wichtig, wenn WordPress-Konten den UPN statt der Mailadresse tragen. Der Abgleich läuft in der Reihenfolge: gebundene Objekt-ID → zugewiesener UPN → Mailadresse → UPN. |
 | Angemeldet bleiben | aus | 14-Tage-Session statt Browser-Session. |
 | Erlaubte E-Mail-Domains | leer | Kommagetrennte Liste, z. B. `contoso.com, contoso.de`. |
 
@@ -322,7 +322,7 @@ automatisch per WP-Cron (stündlich, zweimal täglich, täglich) oder per WP-CLI
 | Situation | Ergebnis |
 | --- | --- |
 | Kein WordPress-Konto vorhanden | Konto wird angelegt: Benutzername aus der E-Mail, Zufallspasswort, **keine E-Mail an den Benutzer**, Standardrolle + zugeordnete Rollen. Die Anmeldung läuft über den Microsoft-Button. |
-| Konto mit derselben E-Mail existiert schon | Wird mit der Microsoft-Objekt-ID verknüpft, Profilfelder werden aktualisiert. Rollen bleiben unangetastet, außer *„Auch die Rollen von Konten verwalten, die schon vor dem Sync existierten“* ist aktiv. |
+| Konto mit derselben E-Mail (Mailadresse **oder** UPN) existiert schon | Wird mit der Microsoft-Objekt-ID verknüpft, Profilfelder werden aktualisiert. Rollen bleiben unangetastet, außer *„Auch die Rollen von Konten verwalten, die schon vor dem Sync existierten“* ist aktiv. |
 | Bereits verknüpft | E-Mail-Adresse, Profilfelder, Profilbild und (bei importierten Konten) Rollen werden aktualisiert. |
 | In Microsoft 365 **deaktiviert** | Wahlweise nichts tun, WordPress-Konto **deaktivieren** oder **löschen**. |
 | In Microsoft 365 **gelöscht** | Wahlweise nichts tun, deaktivieren oder löschen. |
diff --git a/includes/class-m365-login-auth.php b/includes/class-m365-login-auth.php
index c82d9b9..5c23915 100644
--- a/includes/class-m365-login-auth.php
+++ b/includes/class-m365-login-auth.php
@@ -533,22 +533,30 @@ class M365_Login_Auth {
 			$this->link_account( (int) $attempt['link_user'], $claims, $oid );
 		}
 
-		$email = $this->email_from_claims( $claims );
-		if ( '' !== $email && ! $this->domain_allowed( $email ) ) {
+		// E-mail and user principal name may differ: every usable address is tried (on the domain allow-list).
+		$candidates = $this->email_candidates( $claims );
+		$email      = $candidates ? $candidates[0] : '';
+		$allowed    = array_values( array_filter( $candidates, array( $this, 'domain_allowed' ) ) );
+		if ( $candidates && ! $allowed ) {
 			$this->fail( 'domain_not_allowed' );
 		}
 
 		// 1. An account already bound to this Microsoft identity, 2. an account the administrator
-		// assigned this user principal name to, 3. the e-mail address.
+		// assigned this user principal name to, 3. the e-mail address, then the user principal name.
 		$user = $this->find_bound_user( $oid );
 		if ( ! $user ) {
 			$user = $this->find_assigned_user( $claims );
 		}
 		if ( ! $user ) {
-			if ( '' === $email ) {
+			if ( ! $allowed ) {
 				$this->fail( 'no_email' );
 			}
-			$user = get_user_by( 'email', $email );
+			foreach ( $allowed as $candidate ) {
+				$user = get_user_by( 'email', $candidate );
+				if ( $user instanceof WP_User ) {
+					break;
+				}
+			}
 		}
 		if ( ! $user instanceof WP_User ) {
 			/** This action is documented in wp-includes/user.php */
@@ -1072,12 +1080,12 @@ class M365_Login_Auth {
 	}
 
 	/**
-	 * Extracts the e-mail address used for matching.
+	 * Addresses used for matching, in order (e-mail claim, then user principal name).
 	 *
 	 * @param array $claims Verified claims.
-	 * @return string Lowercase e-mail or empty string.
+	 * @return string[] Lowercase addresses.
 	 */
-	private function email_from_claims( $claims ) {
+	private function email_candidates( $claims ) {
 		$candidates = array();
 		$email      = ! empty( $claims['email'] ) && is_string( $claims['email'] ) ? $claims['email'] : '';
 		$upn        = ! empty( $claims['preferred_username'] ) && is_string( $claims['preferred_username'] ) ? $claims['preferred_username'] : '';
@@ -1101,19 +1109,23 @@ class M365_Login_Auth {
 			}
 		}
 
+		$out = array();
 		foreach ( $candidates as $candidate ) {
 			$candidate = strtolower( trim( $candidate ) );
 			if ( is_email( $candidate ) ) {
 				/**
-				 * Filters the e-mail address used to look up the WordPress user.
+				 * Filters an e-mail address used to look up the WordPress user.
 				 *
 				 * @param string $email  E-mail from the token.
 				 * @param array  $claims Verified claims.
 				 */
-				return (string) apply_filters( 'm365_login_match_email', $candidate, $claims );
+				$candidate = strtolower( (string) apply_filters( 'm365_login_match_email', $candidate, $claims ) );
+				if ( is_email( $candidate ) && ! in_array( $candidate, $out, true ) ) {
+					$out[] = $candidate;
+				}
 			}
 		}
-		return '';
+		return $out;
 	}
 
 	/**
diff --git a/includes/class-m365-login-sync.php b/includes/class-m365-login-sync.php
index 258ce87..402fb07 100644
--- a/includes/class-m365-login-sync.php
+++ b/includes/class-m365-login-sync.php
@@ -749,6 +749,11 @@ class M365_Login_Sync {
 			if ( ! $by_mail ) {
 				$by_mail = get_user_by( 'email', $email );
 			}
+			// The WordPress account may use the user principal name instead of the mail address.
+			$upn = self::member_upn( $person );
+			if ( ! $by_mail && '' !== $upn && $upn !== $email && is_email( $upn ) && $this->domain_allowed( $upn ) ) {
+				$by_mail = get_user_by( 'email', $upn );
+			}
 			if ( $by_mail instanceof WP_User ) {
 				$stored = strtolower( (string) get_user_meta( $by_mail->ID, M365_Login_Auth::META_OID, true ) );
 				if ( '' !== $stored && $stored !== $oid ) {
@@ -1294,7 +1299,7 @@ class M365_Login_Sync {
 			return false;
 		}
 		$assigned = strtolower( (string) get_user_meta( $user->ID, M365_Login_Auth::META_UPN, true ) );
-		return ( '' !== $assigned && $assigned === $upn ) || ( strtolower( $user->user_email ) === $upn && $upn === $email );
+		return ( '' !== $assigned && $assigned === $upn ) || strtolower( $user->user_email ) === $upn;
 	}
 
 	/**