My customers receive the following error in Microsoft Exchange Online after an email is forwarded to Zendesk:
550 5.7.360 Remote server returned message denied by administrative policy -> 550 Administrative prohibition - envelope blocked - https://community.mimecast.com/docs/DOC-1369#550
If your account is using mail forwarding from Microsoft email addresses and the forwarding rules are blocked by a Microsoft security setting, your Zendesk Support addresses may seem as if they are rejecting traffic as a result.
To learn more about email forwarding in Microsoft 365, see this article from Microsoft's help center: Control automatic external email forwarding in Microsoft 365.
To resolve this issue, work with your IT email administrator to make the changes required by Microsoft, as outlined in the article: Fix email delivery issues for error code 550 5.7.1 in Exchange Online.