Message wasn’t delivered because the recipient’s email provider rejected it

• Are there any other users who are experiencing the same issue?
• As per the issue description it looks that the recipient is not accepting the mails from your domain or sender.
• You can contact the recipient to check at their end.
• Ask the recipient to Add your domain to their safe senders/Allowed/whitelist list. 

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 domain
• 554 5.7.0 Reject id=xxxxx – SPAM
• 5.7.0 x-unix; maildrop: maildir over quota
• 550 5.7.3 Message classification was not recognized ##rfc822

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:

• the recipient domain doesn’t exist. Check the spelling of the domain and if public DNS (A and MX) records exist
• 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

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:

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

550 5.7.3 Message classification was not recognized ##rfc822

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