In this post, let’s have a look at the error message “5.1.8 – Access denied, bad outbound sender” 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.
5.1.8 – Access denied, bad outbound sender
The account has been blocked for sending too much spam. Typically, this problem occurs because the account has been compromised (hacked) by phishing or malware.
For more information, see Fix email delivery issues for error code 5.1.8 in Exchange Online.