This article will talk about popular errors we can see when attempting to deliver an email to a non customers mail server. These errors would be generated be the recipients side and their Spam Filtering/Content Control gateway.

Please note below are the most popular errors we see from time to time, there are various other errors that can be generated by the recipients Gateway. Different Content Filtering Gateways will produce their own formatted error messages.

• 550 5.7.1 Message rejected as spam by Content Filtering
• 551 Sender is on domain's blacklist
• 5.7.1 Recipient not authorized, your IP has been found on a block list
• 5.7.1 Message Rejected as spam by Content Filter
• 554 Rejected due to banned content
• 554  Rejected due to spam content
• 550 Permission denied
• 550 Connection refused
• 550 "5.7.0" Mail relay denied

Recipient end would need to reach out to their internal teams and possibly their Email Security service provider for them to perform an investigation behind these errors and what might be causing them.