Actually, email can be the hardest thing to do server-wise.
You are going to have to learn about setting up server side spam and virus filters, compiling your own software and hand tweaking some hairy config files.
The good news is that you can do it.
If you are just hosting a single domain with a few mailboxes you shouldn't have much trouble, relatively speaking.
If you haven't been scared off yet then go here :
Stepwise: Email Servers and Mac OS X
Dig in and have fun.
There are some comercial products that can make things much easier.
DMail has a free version for 5 users or less.
go here haven't tried it
Maybe also check out Post.Office and EIMS.
Good luck!