Message Not Delivered: Unknown issue with email address

The message could not be delivered because there is a problem of some sort with the recipient's email address.

Fixes

Check 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 necessary, contact the recipient another way (e.g., phone or text message) to confirm their email address.

5.1.0: Official Definition

[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.

[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.1.0:

Something about the address specified in the message caused this DSN.

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