You probably can't run it yourself as it integrates fully with Active Directory, just about everything is stored in AD, besides the mail, that is in the Exchange mailstore(s) DB.
I tried Hosted Exchange (Exchange made available through ISP's) and it performed bad.
Some ISP offer hosted exchange. You do not maintain exchange yourself, you just get a number of mailboxes to which you can connecto to over the Internet from outlook, not only through POP. But MAPI as well.
Bad performance might be due to the fact I was using a trial account to test it out. Hosted Exchange has its benifits, you get the whole mailbox functionality, like calendering etc.
Search for "Hosted Exchange" using Google to find out more.
Danny