To do that though, the OP will need to send his (corporate?) email to mail2web, which may be a no-no, a la the Lotus Notes thread recently. As far as I can tell in WM5, there's no way to have your inbox be from the POP source and use an Exchange account as the repository for sent items.
If you're doing this with personal e-mail, or you run your own shop, this shouldn't be a problem though - I have my personal e-mail automatically mirrored all over the place (it downloads from Yahoo and is stored on an IMAP-accessible server on my home network, forward a copy to GMail, and forward a copy to mail2web to get it on my phone. (not that I need it, but just because I can)
I did a little googling, and there might be a commercial app that does what you need - take a look at FlexMail at
http://www.webis.net/products_info.php?p_id=mail and see if it might be suitable. I have no experience with the software, but reading the description on the site, it sounds like it could accomplish what you need. There is a free trial version, $29.95 to buy.