Undeliverable E-Mail - SMTP error from remote mail server host

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

 
    host mail.ipxo.com[104.26.4.35]
    SMTP error from remote mail server after 
    550 5.1.1 <> recipient domain rejected - invalid DNS
Reporting-MTA: dns; box2241.mailserver.com

Action: failed
Final-Recipient: rfc822;
Status: 5.0.0
Remote-MTA: dns; .mail.ipxo.com
Diagnostic-Code: smtp; 550 5.1.1 <> recipient domain rejected - invalid DNS