To prevent open-relay customers from compromising HES relay servers, HES relay MTA will no longer accept and forward mails from senders who use non-registered domain(s) in the sender from email address.

When HES relay servers receive an email from a customer, HES checks if the sender domain (which is taken from the envelope "mail from") matches the relay domain ACL and does the following:

• If yes, HES receives the mail for scanning and forwards it to the intended recipient.
• If not, HES rejects the email and sends back a permanent error.