554 5.2.3 MailPolicy violation Error delivering to mailboxes error

554 5.2.3 MailPolicy violation Error delivering to mailboxes error

Hi,

A client of ours has received a bounceback when sending to our @therestartproject.org email domain.
The following error was in the bounceback:


The response from the remote server was:

554 5.2.3 MailPolicy violation Error delivering to mailboxes


Please could you let me know what the reason was for this bounceback?

And could you also tell me if any other bouncebacks have occurred on our domain.

Kind regards

Neil