Delayed Message: User name in email address not valid
The email message has not yet been delivered because the mailbox for the recipient's email address is invalid or does not exist.
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.
4.1.1: Official Definition
A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
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.