This happen if the Postfix cannot deliver mail to the relay server, and also cannot send the undeliverable report to the original sender.
- In case there is no recipient verification process for incoming mails in the front of the MTA. The Postfix does not know if the recipient exist or not. (It is not recommended to act as first contact for incoming mails.)
- The Check Point Mail Transfer Agent (MTA) accept all mails without any restriction.