If a mail is undeliverable it's bounced and the account should be flagged somehow for an email address update. Most mailing lists just unsubscribe the user depending on the error code. It my case I'd like to see the bounced user moved to the awaiting email confirmation usergroup.
Action: failed
Status: 5.0.0 (permanent failure)
Remote-MTA: dns; [000.000.000]
Diagnostic-Code: smtp; 5.1.0 - Unknown address error 550-'Requested action not taken: mailbox unavailable' (delivery attempts: 0)
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