Diagnostic-Code: smtp; 554 5.2.3 MailPolicy violation Error delivering to mailboxes

Diagnostic-Code: smtp; 554 5.2.3 MailPolicy violation Error delivering to mailboxes

Starting last Friday, October 23, I've received two emails with the following error:

An error occurred while trying to deliver the mail to the following recipients: 
(email removed for privacy)-MTA: dsn; a8-81.smtp-out.amazonses.com 

Action: failed 
Final-Recipient: rfc822; (email removed for privacy)
Diagnostic-Code: smtp; 554 5.2.3 MailPolicy violation Error delivering to mailboxes 
Status: 5.2.3 

These emails are automated and I received them daily from amazonses.com. Everything has worked fine for months, so i'm not sure why I've started to have this issue, because I've changed nothing. So i think the issue is on zoho's end.