The error that the other server returned was: 554 5.2.3 MailPolicy violation Error delivering to mailboxes

The error that the other server returned was: 554 5.2.3 MailPolicy violation Error delivering to mailboxes

Hello,

I am wondering what happen? I have not send any email this few days and got a feedback from my client that emails are being bounced.

The error " The error that the other server returned was:
554 5.2.3 MailPolicy violation Error delivering to mailboxes".

However, I tried to replicate it from my GMail account but I do not get this error.

Please advise.

Regards,
Hamizan