I don't think "another" system would work for you, since it's specifically MS Exchange that the Pre is looking for (I suspect it's ActiveSync, and other solutions likely won't have that).
If you really want to go this route, I'd suggest you get on eBay and look for Small Business Server 2003. It includes Exchange 2003 and 5 cals. You can probably pick up a copy for about $200. Dig out an old PC, and install it there.
I'm still pondering what I want to do. I already have Exchange set up (using SBS 2003), but I've never started using it. I've started looking at it closer. Then I started considering Google Apps ($50 a year is more than I would likely spend on SPAM filtering if I start bring my email to my local server). Plus, I'm still holding out for what I envisioned when I wrote about "
bringing the cloude home".
I'm probably going to wait until the Pre is in my hands, sign up for the 30 day trial of Google Apps, and redirect my mail there for a month. Then I'll see if the solution I really want comes out soon.