fix for issue nextcloud#11281

config/config.sample.php

@@ -421,6 +421,20 @@
  */
 'mail_send_plaintext_only' => false,
 
+/**
+ * Which mode is used for sendmail/qmail: ``smtp`` or ``pipe``.
+ *
+ * For ``smtp`` the sendmail binary is started with the parameter ``-bs``:
+ *   - Use the SMTP protocol on standard input and output.
+ *
+ * For ``pipe`` the binary is started with the parameters ``-t -oi``:
+ *   - Read message from STDIN and extract recipients.
+ *   - Ignore dots.
+ *
+ * Defaults to ``smtp``
+ */
+'mail_sendmailmode' => 'smtp',
+
 /**
  * Proxy Configurations
  */

lib/private/Mail/Mailer.php

@@ -278,6 +278,15 @@ protected function getSendMailInstance(): \Swift_SendmailTransport {
 				break;
 		}
 
-		return new \Swift_SendmailTransport($binaryPath . ' -bs');
+		switch ($this->config->getSystemValue('mail_sendmailmode', 'smtp')) {
+			case 'pipe':
+				$binaryParam = ' -t -oi';
+				break;
+			default:
+				$binaryParam = ' -bs';
+				break;
+		}
+
+		return new \Swift_SendmailTransport($binaryPath . $binaryParam);
 	}
 }