554 5.2.3 MailPolicy violation or something else

554 5.2.3 MailPolicy violation or something else

Hi, I have GMail autoforwarding to Zoho Mail but have been getting some undeliverable messages in GMail recently.

Headers say 

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain zoho.com by mx.zohomail.com. [72.5.230.114].

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

Would this be because there were large attachments or something else?

Thanks