However below are the explanation for each of the possible scenarios.
Message wasn’t delivered because the recipient’s email provider rejected it
Means that the message was rejected by one of the servers trying to deliver it.
The Diagnostic information for administrators section may contain the following errors:
550 5.7.0 Proxy session setup failed on Frontend with ‘554 5.4.4 SMTPSEND.DNS.NonExistentDomain; nonexistent domainIt may be caused by the following issues:
All messages sent to large mail providers (e.g. gmail.com, yahoo.com, etc.) are scanned when leaving our system in order to prevent spam from being sent using our servers. This error message indicates that the message was filitered and rejected as it got a very high spam score.If the message was legitimate, contact Support and provide them with the full bounce back message (including the Diagnostic infotmaion for administrators section)Note: the same error message may show up in the case of the following non-delivery report:Your message wasn’t delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator
5.7.0 x-unix; maildrop: maildir over quota
It indicates that the recipient POP mailbox has reached storage limits.To resolve the issue:
The error message indicates that Exchange server was unable to recognized message classification. If you’re not using any custom classification, that usually means that the message is somehow corrupted and there is something in the message content that is preventing delivery.
To resolve the issue, copy and paste the text of the email to a new message. Should that not work, contact Support