Communigate Pro is a mail server that runs native on the as400(Iseries, i ,whatever it's called this week).
We have been using it as our email server for over 7 years. It is truly a set and forget email server. It has a web based interface for administration and email access. We use pop3 access and thunderbird email client rather than web based, but both options are available.
It is surprisingly low priced for small configurations.
Communigate may have some products that require linux, but the email server is native 400.
I am a very satisfied customer and just wanted to set the record straight that Linux is not needed.
---Dale