XGEMAIL_0003 Command rejected (in reply to end of DATA command)

Hi list,

from today when sending emails to one of our supplier we receive:

Arrival-Date: Mon,  1 Nov 2021 11:19:08 +0100 (CET)

Final-Recipient: rfc822; user@userdomain.com
Original-Recipient: rfc822;user@userdomain.com
Action: failed
Status: 5.7.1
Remote-MTA: dns; mx-01-us-east-2.prod.hydra.sophos.com
Diagnostic-Code: smtp; 550 5.7.1 XGEMAIL_0003 Command rejected

Doesn't matter if sended using our email server or from gmail, same result.

Any clue ?

Daniel