feat(sharebymail): Use initiator's email as sender if mail providers enabled

apps/sharebymail/lib/ShareByMailProvider.php

@@ -380,7 +380,7 @@ protected function sendEmail(IShare $share, array $emails): void {
 				]
 			);
 		}
-		$message->setFrom([Util::getDefaultEmailAddress($instanceName) => $senderName]);
+		$message->setFrom([Util::getEmailAddressForUser($initiatorUser, $instanceName) => $senderName]);
 
 		// The "Reply-To" is set to the sharer if an mail address is configured
 		// also the default footer contains a "Do not reply" which needs to be adjusted.

lib/public/Util.php

@@ -314,6 +314,38 @@ public static function getDefaultEmailAddress(string $user_part): string {
 		return $user_part . '@localhost.localdomain';
 	}
 
+	/**
+	 * Returns the email address for the given user
+	 * @param IUser|null $user
+	 * @param string $fallback_user_part the fallback address part
+	 * @return string the email address
+	 *
+	 * If mail providers are enabled, uses the specified user's email address.
+	 * If disabled or the user has no valid email, falls back to the system default.
+	 * @since 31.0.12
+	 */
+	public static function getEmailAddressForUser(?IUser $user, string $fallback_user_part): string {
+		if ($user === null) {
+			return self::getDefaultEmailAddress($fallback_user_part);
+		}
+
+		$config = \OCP\Server::get(serviceName: IConfig::class);
+
+		$mailProvidersEnabled = $config->getAppValue('core', 'mail_providers_enabled', '0') === '1';
+
+		if ($mailProvidersEnabled) {
+			$userEmail = $user->getEMailAddress();
+
+			if ($userEmail !== null) {
+				$emailValidator = \OCP\Server::get(IEmailValidator::class);
+				if ($emailValidator->isValid($userEmail)) {
+					return $userEmail;
+				}
+			}
+		}
+		return self::getDefaultEmailAddress($fallback_user_part);
+	}
+
 	/**
 	 * Converts string to int of float depending on if it fits an int
 	 * @param numeric-string|float|int $number numeric string