Delivery Subsystem / Message Delivery Failure / The follwing recipient(s) could not be reached

When you receive one of these, look in the message for the reason, and (if present) also look for the reason code, and error number and check the corresponding section below:

Reason: Remote SMTP Server Returned: 550 5.1.1 unknown or illegal alias

Reason Code: Invalid Domain Name, Error Number: 9003

Reason: Remote SMTP Server Returned: 550 Requested action not taken: mailbox unavailable

Reason: Remote SMTP Server Returned: 550 MAILBOX NOT FOUND