Cpanel - Incoming email not working

Job ID: 31919464

Budget: $10 – $30 USD

Incoming emails stopped working in one of our cpanel servers.

We can't see what happens:

---------- Forwarded message ---------
De: Mail Delivery System <Mailer-Daemon.at.l1.ourcompany.net>
Date: lun, 25 oct 2021 a las 17:32
Subject: Mail delivery failed: returning message to sender
To: <user.at.gmail.com>


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:

test.at.clientdomain.com
Related categories: PHP Linux Software Architecture DNS Email Handling