That, on the other hand, gets you into trouble with rather stupid Spam filters, that only accept mails from a server, if that server is also MX for the senders domain.
Yes, this is stupid, but that does not change the fact, that these setups are out there.
No, they're not. Large ISPs, starting with AOL and Yahoo, separated their inbound and outbound mail servers years ago. Anyone who still uses "mail from MX" for filtering doesn't really care if he gets mail or not. Note that this is a different issue from separating your public inbound MX servers from your user-only submit servers. I've done that, too, and haven't had any problems other than educating the occasional too-clever user who thinks my setup instructions must be wrong, substitutes the MX server for the SUBMIT server, and then complains that it doesn't work. Regards, John Levine, johnl@iecc.com, Primary Perpetrator of "The Internet for Dummies", Information Superhighwayman wanna-be, http://www.johnlevine.com, Mayor "I shook hands with Senators Dole and Inouye," said Tom, disarmingly.