I do not know if this is related to the recent issues with the zoho mail server because we've been experiencing this all week and occasionally throughout the past six months. Searching through the forums I see that this has been a reoccurring issue with desk for years.

We're getting messages from  mailer-daemon@mail.zoho.com with an assortment of errors related to email addresses. They all suggest that the outgoing email does not need to be re-sent.

Ex:

It has happened with addresses of multiple domains, so it does not appear to be a problem with the mail server of any one domain with whom we're corresponding. Please see the attached messages.