The Sentinel Messaging MTA’s will give a 550 user invalid response to inbound email sent to a recipient address that has been marked as Invalid by the SMTP Discovery service.
“550 User email address is marked as invalid”
If the email address is in fact required, it should be removed from the Invalid list.
Invalid email addresses are summarized in our SMTP Discovery reports, which have been running weekly since Jan 2011.
See: SMTP Discovery for more information on SMTP Discovery reports
To minimize administration requirements and to ensure an accurate email list for future reference, please configure LDAP discovery from the Sentinel Messaging Organization Administration area.