In this post, let’s have a look at the error message “5.7.134 – Sender was not authenticated for mailbox” 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.
5.7.134 – Sender was not authenticated for mailbox
The recipient address is a mailbox that is set up to reject messages sent from outside of its organization. Only an email admin for the recipient’s organization can change this.
For more information, see Fix email delivery issues for error code 5.7.134 in Exchange Online.