I started my sysadmin career long ago by digesting the Sendmail Bat Book in a day. I've messed with Exim on systems that had it installed by default. And I've set up Postfix to handle multiple domains on a server. Multiple times.
So believe me when I tell you that Postfix is the only way to go while retaining your sanity.
(It helps if you can tell the client to use a port other than 25, so you can put different rules on 25 than on the "submission" port.)
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I've messed with Exim on systems that had it installed by default.
And I've set up Postfix to handle multiple domains on a server. Multiple times.
So believe me when I tell you that Postfix is the only way to go while retaining your sanity.
(It helps if you can tell the client to use a port other than 25, so you can put different rules on 25 than on the "submission" port.)