5.1.1

Message Not Delivered: user name of email address not valid

The message could not be delivered because the mailbox for the recipient's email address is invalid or does not exist.

Fixes

Check the username part of the recipient's email address for misspellings or missing letters. If an error is found, correct it in contacts list or address book for next time.

If the address looks correct, there might be an issue with the recipient's domain. If necessary, contact the recipient another way (e.g., phone or text message) to confirm their email address. Or contact your mailbox provider for help.

5.1.1: Official Definition

5.X.X

[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 5.X.X:

A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.

X.1.1

[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.1.1:

The mailbox specified in the address does not exist. For Internet mail names, this means the address portion to the left of the "@" sign is invalid. This code is only useful for permanent failures.