What kind of mail server do you use?

Postfix ... and for the why http://www.redhat.com/support/resources/howto/RH-postfix-HOWTO/c49.html

Exim + Exchange.

Exchange is a pretty good groupware client, and advantage No.1[*] is Activesync (email + calendar sync) which frees us from relying on Blackberry :)

Having said that, I wouldn't make an exchange server directly internet facing as the bulk of mail received by a normal corporate MTA is invalid (spam etc) and the cost per message of running an efficient exchange server is much higher that a solid *nix based MTA.

Exim is (IMHO, and as mentioned) the most flexible, stable and resource efficient way of handling large or small volumes of email - you can have spam/virus filtering with a few additional apps and any rules based flexibility required using the exim filter system. I have never come across a mail requirement that couldn't be handled by exim (and I've had some strange requirements) and the documentation and mailing list support is great.

My usual setup is Internet -> Exim -> internal Exchange server with a bit of NAT for active sync and outlook web access. Scalable, secure & easy to manage.

[*] for me. I think other people like it for other reasons... :)

Ubuntu 8.04 server + Postfix + Dovecot. IMAP clients. Reason is KISS. Easy to set up and care+feeding is minimal. Modest hardware requirements.

We use Zimbra. It's much cheaper than Exchange, uses open source technologies, the Desktop client works on Windows, Linux and Non-PPC Mac's.

The server portion is also a very easy install on a Debian box, but runs on Windows as well.