In this post, let’s have a look at the error message “5.7.1 – Unable to relay” that you get when working with Microsoft 365 Exchange Online.
These are the errors that you usually receive when there is a problem in delivering the email message that was sent by you. These errors are generated and sent by Office 365. This is usually alled as DSN which stands for delivery status notification or in simple words bounce messages.
NDR or Non-delivery report is one of the common type of message that you receive which indicates to the user that the email was not delivered.
5.7.1 – Unable to relay
The sending email system isn’t allowed to send a message to an email system where that email system isn’t the final destination of the message.
This error occurs when the sending email system tries to send an anonymous message to a receiving email system, and the receiving email system doesn’t accept messages for the domain or domains specified in one or more of the recipients. The following are the most common reasons for this error:
A third party tries to use a receiving email system to send spam, and the receiving email system rejects the attempt. By the nature of spam, the sender’s email address might have been forged, and the resulting NDR could have been sent to the unsuspecting sender’s email address. It’s difficult to avoid this situation.
An MX record for a domain points to a receiving email system where that domain isn’t accepted. The administrator responsible for the specific domain name must correct the MX record or configure the receiving email system to accept messages sent to that domain, or both.
A sending email system or client that should use the receiving email system to relay messages doesn’t have the correct permissions to do this.
For more information, see Fix email delivery issues for error code 5.7.1 in Exchange Online.