Rejection errors when delivering to 3rd party recipient server

Products

• Email Security.cloud

Issue/Introduction

An outbound email sent to a third-party recipient which resulted in a bounce back indicating that the mail delivery agent was unable to deliver your message. 

The error codes listed below include some of the most common error messages the recipient's mail server could return.

I was unable to deliver your message to the following addresses:

[e-mail]

• Reason: 550 5.7.1 Rejecting mail, <originating IP address> is blacklisted
• Reason: 550 unknown user
• Reason: 550 Denied by policy
• Reason: 521 access denied
• Reason: 553 Relay denied
• Reason: 554 Too many Received: headers
• Reason: 550 Message rejected due to sender IP reputation ([xx.xx.xx.xx])

Environment

Email Security.cloud

Resolution

This indicates, that we have attempted to deliver the message to the intended recipient’s server, but it was not accepted by that server. The solution depends on the reason provided by the recipient server.  It may be an invalid address, the recipient may be blocking the mail based on content or address, or the recipient system may be misconfigured. Follow up as to the specific reason must be with the owner of the recipient server.