Message Not Delivered: Email address malformed
The message could not be delivered because the recipient's email address is not a valid email address.
Fixes
Check the recipient's email address for spaces or unusual characters. 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.
5.1.3: 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.1.3:
The destination address was syntactically invalid. This can apply to any field in the address. This code is only useful for permanent failures.