554 5.2.3 Mailpolicy violation error delivering to mailboxes

554 5.2.3 Mailpolicy violation error delivering to mailboxes

Dear Srs.

I have a client that is trying to send email to my custom account and is receiving this message " 554 5.2.3 Mailpolicy violation error delivering to mailboxes".
Please can ZOHO solve this problem for me?
What is causing this type of problem in my account?

Regards,
Guido