Easy peasy mail
Bah, I really want to mention my employers solution to this (a FLOSS project to make debian + exim = other things easier) but I'm concerned about coming across as a bit shilly.
I'll waffle on about it a bit if someone wants me to, but suffice to say my entire (pitiful) online estate runs on it these days, not because I have to, but because it's really rather good in my humble opinion. We have some fairly hefty servers running dozens of domains using it, too, because it works nicely and tends to Not Break and is easy to configure - a nice mix.
Steven "Google Bytemark Symbiosis, kids" R