In this post, let’s have a look at the error message “5.7.1 – Client was not authenticated” 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.
Error Message
5.7.1 – Client was not authenticated
Error Details
The sending email system didn’t authenticate with the receiving email system. The receiving email system requires authentication before message submission.
This error occurs when the receiving server must be authenticated before message submission, and the sending email system hasn’t authenticated with the receiving email system. The sending email system administrator must configure the sending email system to authenticate with the receiving email system for delivery to be successful.
For more information, see Fix email delivery issues for error code 5.7.1 in Exchange Online.
