Back-end mail system

I admit to being a bit ignorant as to the internals of SmarterMail, but there was something I'm curious about. Does it have it's own back-end storage mechanism for e-mail or is it just a front end for something like Exchange Server? Reason I ask is, since you guys are such a great Microsoft shop I wondered if you're backing mail with Exchange server and if so, what the limitations would be preventing using Outlook Web Access as the web interface. I'm guessing licensing costs would be prohibitive and there might be technical challenges hosting multiple domains, but thought I'd ask nonetheless because I've never seen a better web-based mail interface than OWA (in my opinion anyway as an Outlook junkie).