As has previously been written, the Raspberry Pi has limited performance. It just doesn't make sense to have it do heavy lifting, so best make do with lightweight stuff whereever possible. Certainly, there's no point in putting in unnecessary stuff such as a graphical interface or a kitchen sink.
A full-blown apache webserver may be overdoing it, so we go for lighthttpd. postfix is pretty lightweight for a full-featured mailserver. MySQL should do even on a low-performance box.