postfix+cyrus is the way to go. It has a bit steep learning curve (on the cyrus side, postfix is easy) but it's the best you can get.
I won't go into details, but if you can build one with those you will understand how email works and what is important.
FYI, I've desingned, implemented, integrated and debugged e-mail systems that have to scale to hundreds of thousands of users. Cyrus+postfix does the trick without any hiccups and is easy to support. (of course in large systems you scale this up but basically...)
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