Don't know if they are easy, but I know about the following (free) mail servers:
- Afterlogic's XMail (available for both Linux and Windows) (3Mb).
- SymPa (Windows) (1Mb)
This one I do know:
- vpop3 (Windows) Small and very able server (600Kb)(45 USD)
It uses a mailbox from your ISP and hooks up an n amount of virtual virtual mailboxes to that one.
Works like a charm with the default settings, good documentation and a boatload of extra functionality at your fingertips.
At the time I was using it (on a 128Mb RAM Win2000 server) it had a small footprint. Well worth the money.