RFC 5321 says that if a mail server gives an initial banner with a 554 status code, that means "no mail server here", so the client should do whatever it normally does on a connection failure, looking for another MX at equal or lower priority.
This is different from 554 later in the SMTP session, which means that an address or message is rejected. What does Postfix do when it gets a 554 code on the initial connection?
