Would probably be better to use IMAP for this. Then it will work for Exchange systems, GMail, and nearly every other mail system out there.
Unfortunately, I don't think IMAP isn't enabled by default in Exchange. And, at least at my work, enabling it isn't an option.
Why isn't it an option? If you work in the sort of environment that wont enable IMAP, you probably also work in the sort of environment that doesn't want you giving up access to your mailbox to a random third party.
Using ActiveSync to access Exchange would probably be the best way to pull it off (though there are probably patent-related concerns). A lot of Exchange admins aren't going to like opening up IMAP, but nearly every Exchange installation today has ActiveSync enabled and exposed to the Internet.