Question : Good, reasonablly priced alternative to Exchange

We are looking into migrating from Exchange to a much cheaper alternative.
We have approximately 1,200 users which will connect to the new server.  These users will only need basic email access.  They will connect up to the email server via pop3 without the need for any of the features in Exchange such as the Global Address list and such.

My question is, what are the best available options out there for our scenario?  1,200 users with the need for basic email access via pop3 using Outlook Express (preferrably.)

So far in the pot of software to test I have:
- OpenGroupware
- Scalix
- Zimbra


Any further recommendations, or expert opinions either the ones listed or other options??

Answer : Good, reasonablly priced alternative to Exchange

What you basicaly need is the following (no costs at all):
SMTP: e.g. exim or postfix
POP3: e.g. dovecot or cyrus mail system
its open source I use (linux world). But at least exim runs also on windows using the cygwin environment.
When the OS could be linux I would recommend a debian lenny based system (easy install, easy administration, easy security updates).

Regards,

Oli