MailPolicy Violation

Can you please check all the mailboxes under our domain @muber.com.au. I tried to send some images to some of the users and received the following error messages.

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain muber.com.au by mx.zoho.com . [165.254.167.50].

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

The email addresses were :
barry@muber.com.au
becci@muber.com.au
manne@muber.com.au
allie@muber.com.au

Can you also please explain why I received these error as I don't think we violated any of your policy. Thanks and look forward to your prompt reply.

Regards,

Allie Syadiqin
IT Manager
MUBER Pty Ltd.