Questions about mail services

My company is looking for an email solution to replace our 3rd pary mail system (currently runs on Windows). I was thinking about putting in a Xserve running mail services, but I don't know much about it. My requirements are:

1. Support for up to 200 POP3 OR IMAP accounts
2. Ability to foward mail (message goes to recipient and then is forwarded to a different account on a different server).
3. Support for MS Outlook

Optional things that might be nice

1. Ability to query or connect to a LDAP server. We run AD right now.

Has anyone implemented something like this? Would it work for me?

An Exchange solution will be way too expensive.

When you say encrypted options, what do you mean? Which protocols?

My users can use Thunderbird or another email client. They don't need Outlook.

Can the OSX mail server "store and forward"? In other words, if a message comes in, can it forward to another external account but keep a copy on the server?

Linux is a possibility, but there are two problems with it:

1. No support (really don't want to be chatting in IRC with some 16 year old kid about the weird error I'm getting because there is skimpy documentation and no corporation backs the software).

2. Backing up Linux boxes is an issue. There is an OSX agent for Backup Exec I can use to back up the Xserve to tape.

I have Exchange running for some of my users right now, but it sucks. It is a resource hog, and is virtually impossible to restore if something goes seriously wrong. If the server died, it would take me two weeks to get the thing back online, thanks to all the AD integration, Kerberos time-stamp issues, certificates, etc.