An external domain is trying to send emails to our Symantec Mail Security for SMTP gateway server which handles our inbound email from the internet. The emails are then forwarded to an Exchange frontend then to the backend server. Their users are recieving the following NDR when trying to email users in our domain. Failed to deliver to 'test@mydomain.com' SMTP module(domain mydomain.com) reports: message text rejected by mail1.mydomain.com: 550 Requested action not taken: NUL characters are not allowed. Would the issue be on their server which I am leaning towards or our gatway?
