Just to make it clear, RFC2821 is likely being violated: - The reserved mailbox name "postmaster" may be used in a RCPT command without domain qualification (see section 4.1.1.3) and MUST be accepted if so used. This is *probably* being violated, but I'm not sure as it gets ambiguous later. However, returning a 550 is very wrong. The correct reply should be either: 554 Transaction failed (Or, in the case of a connection-opening response, "No SMTP service here") or: 502 Command not implemented