Mail delivery failed: returning message to sender errors are generally encountered when an intended recipient does not exist on the remote server.
You will normally receive a bounceback message with this error in the subject, and the body of the message should contain the original message that was attempting to be sent.
Example bounce-back message
Return-path: <>
Envelope-to: sender@example.com
Delivery-date: Thu, 25 Oct 2012 16:42:54 -0400
Received: from mailnull by ecbiz103.mazeweb.com with local (Exim 4.77)
id 1TRUGT-0005Qd-RT
for sender@example.com; Thu, 25 Oct 2012 16:42:54 -0400
X-Failed-Recipients: recipient@RemoteDomain.com
Auto-Submitted: auto-replied
From: Mail Delivery System <Mailer-Daemon@ecbiz103.mazeweb.com>
To: sender@example.com
Subject: Mail delivery failed: returning message to sender
Message-Id: <E1TRUGT-0005Qd-RT@ecbiz103.mazeweb.com>
Date: Thu, 25 Oct 2012 16:42:53 -0400
recipients. This is a permanent error. The following address(es) failed:
No Such User Here
Received: from sender@example.com by ecbiz103.mazeweb.com with local (Exim 4.77)
(envelope-from <sender@example.com>)
id 1TRUGT-0005QT-Q5
for recipient@RemoteDomain.com; Thu, 25 Oct 2012 16:42:53 -0400
To: recipient@RemoteDomain.com
Subject: [example.com] Please moderate: "About us"
X-PHP-Script: example.com/wp-comments-post.php for 123.123.123.123
Date: Thu, 25 Oct 2012 20:42:53 +0000
From: WordPress <wordpress@example.com>
Message-ID: <82f2c6cb759fb7e7c03356a25c799790@example.com>
X-Priority: 3
X-Mailer: PHPMailer 5.1 (phpmailer.sourceforge.net)
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="UTF-8"
http://example.com/
E-mail : payday@spam.com
URL : http://www.paydayspam.com
Comment: Payday loans are available for any occasion!
Resolving Mail delivery failed: returning message to sender bounce-backs
Typically these type of bounceback errors are hard bounces meaning that trying to send the message again won't be successful. You can read our previous article on why does email bounce, bounceback, or error? for a more in-depth explanation of a hard bounce.
In the particular bounce-back example above, the cause of the issue is that this user has WordPress setup to allow comments on posts. A spammer attempted to leave a comment on one of their posts, and the WordPress administrator of this site has setup their e-mail account as recipient@RemoteDomain.com. So that is the address used to send comment moderation requests to. However that e-mail address doesn't exist on the RemoteDomain.com mail server, so it's always going to fail and cause a bounce-back error.
If you happen to also be having issues with WordPress comment spam causing bounce-back errors for your account, we have an advanced article on WordPress comment spam clean up that could help you clear this issue up.