Message Not Delivered: Sender's address is written incorrectly
The message could not be delivered because the sending address is not a valid email address.
Fixes
Review the sender's email address to identify any misspellings or missing letters.
In the settings or options of the sender's email program or webmail service, ensure that your own email address is entered correctly.
5.1.7: 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.1.7:
The sender's address was syntactically invalid. This can apply to any field in the address.