Zend certified PHP/Magento developer

Mail delivery failed: returning message to sender

Email sent to a domain are never delivered and my customer receive back an email with:

-----Message d'origine-----
De : Mail Delivery System  
Envoyé : 17 octobre 2019 14:39
À : meldon@SENDERDOMAINE.ca
Objet : Mail delivery failed: returning message to sender

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:

  f.ray@DOMAIN.com
  a.cole@DOMAIN.com

And in an attachment there is a txt file containing:

Reporting-MTA: dns; us205.siteground.us

Action: failed
Final-Recipient: rfc822;a.cole@DOMAIN.com
Status: 5.0.0

Action: failed
Final-Recipient: rfc822;f.ray@DOMAIN.com
Status: 5.0.0

Any idea what’s going on?