Had a 501 error on a bulk email for a couple of email addresses, but unfortunately the message:
"Failed to add recipient: @localhost [SMTP: Invalid response code received from SMTP server while sending email. This is often caused by a misconfiguration in Outbound Email settings. Please verify the settings at Administer CiviCRM >> Global Settings >> Outbound Email (SMTP). (code: 501, response: Address Syntax Error in <@localhost>)]"
was too long for civicrm_mailing_event_bounce.civicrm_mailing_event_bounce which was varchar to 255. Mail queue failed, and cron kept trying to send overnight, rehitting the same people. Oops.
Fixed the error by extending the characters in the field - is this a bug that needs reporting, or just an issue with our install? Still on 4.1.0 at the moment, so it may have been picked up in later version.