Delayed Message: Sender's address is written incorrectly
The email message has not yet been 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.
4.1.7: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 4.X.X:
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.
[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.