Message Not Delivered: Valid email address
The message could not be delivered even though the recipient's email address is a valid email address.
Fixes
Contact the recipient another way (e.g., phone or text message) to confirm their email address.
If the email address you're using is correct and your recipient can receive emails to their inbox, then you could contact the email provider for help.
5.1.5: 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.1.5:
This mailbox address as specified was valid. This status code should be used for positive delivery reports.