The following non-delivery report:

Your message wasn't delivered because the recipient's email provider rejected it
Your message wasn't delivered

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 domain

Proxy session setup failed

This error may be caused by the following issues:

  • the recipient domain doesn't exist
    If you are sure that the domain name was spelled correctly, check if there are DNS records (A and MX) published for the domain to verify that it exists.
  • the sending server is unable to resolve the DNS records of the domain
    If you can look up the DNS records of the recipient domain from your own machine, contact Support.

554 5.7.0 Reject id=xxxxx - SPAM
5.7.0 Reject - SPAM

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 filtered 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 information for administrators section).

Note: this error may also show up with 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

Security policies

5.7.0 x-unix; maildrop: maildir over quota
Maildir over quora

This error indicates that the recipient POP mailbox has reached storage limits.

To resolve the issue:

  • delete or archive some of the data in the mailbox
  • or increase the Receive limit on the Services > Mailboxes > POP/IMAP Email > username > Storage management page:
    Storage management

550 5.7.3 Message classification was not recognized ##rfc822
Message classification was not recognized

This error indicates that the Exchange server was unable to recognize 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.