diff --git modules/system/system.mail.inc modules/system/system.mail.inc
index 181c3c4..7e8c618 100644
--- modules/system/system.mail.inc
+++ modules/system/system.mail.inc
@@ -73,7 +73,7 @@ class DefaultMailSystem implements MailSystemInterface {
         $mail_body,
         $mail_headers,
         // Pass the Return-Path via sendmail's -f command.
-        '-f ' . $message['Return-Path']
+        '-f' . $message['Return-Path']
       );
     }
     else {