Returned mail: see transcript for details

from Mail Delivery Subsystem
----- The following addresses had permanent fatal errors -----
(reason: 550-5.7.1 This message does not have authentication information or fails to pass)
----- Transcript of session follows -----
.... while talking to gmail-smtp-in.l.google.com.:
>>> DATA
<<< 550-5.7.1 This message does not have authentication information or fails to pass
<<< 550-5.7.1 authentication checks. To best protect our users from spam, the
<<< 550-5.7.1 message has been blocked. Please visit
<<< 550-5.7.1 https://support.google.com/mail/answer/81126#authentication for more
<<< 550 5.7.1 information. w124si5200580pfw.62 - gsmtp
554 5.0.0 Service unavailable

Delivery Status Notification (Failure)

The following message to <email address> was undeliverable.
The reason for the problem:
5.1.0 - Unknown address error 550-'cuda_nsu 5.1.1 <email address>: Recipient address rejected: User unknown in virtual mailbox table'