Microsoft 365 Exchange Error Code 5.1.1 – Bad destination mailbox address

In this post, let’s have a look at the error message “5.1.1 – Bad destination mailbox address” 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

5.1.1 – Bad destination mailbox address

Error Details

This failure might be caused by the following conditions:
The recipient’s email address was entered incorrectly by the sender.
No recipient’s exists in the destination email system.
The recipient’s mailbox has been moved and the Outlook recipient cache on the sender’s computer hasn’t updated.
An invalid legacy domain name (DN) exists for the recipient’s mailbox Active Directory Domain Service.

This error typically occurs when the sender of the message incorrectly enters the email address of the recipient. The sender should check the recipient’s email address and send again. This error can also occur if the recipient email address was correct in the past but has changed or has been removed from the destination email system.

If the sender of the message is in the same organization as the recipient, and the recipient’s mailbox still exists, determine whether the recipient’s mailbox has been relocated to a new email server. If this is the case, Outlook might not have updated the recipient cache correctly. Instruct the sender to remove the recipient’s address from sender’s Outlook recipient cache and then create a new message. Resending the original message will result in the same failure.

For more information, see Fix email delivery issues for error code 5.1.1 through 5.1.20 in Exchange Online.