554 5.2.3 MailPolicy violation

554 5.2.3 MailPolicy violation

I'm trying to help my client send emails from @critech.com to support@zohomail.com. However, they are failing with:

host mx.zohomail.com[xxx.xxx.xxx.xxx] said: 554 5.2.3 MailPolicy violation Error delivering to mailboxes

I do not see why critech.com emails are being rejected - please advise how I should solve this issue.

Thanks.