This post on Working Smart details how the author was able to connect to his company’s Exchange server without using VPN. From the post:
I connect directly to my Exchange Server by inserting the address for my Outlook Web Access Web site into the server field of my Account settings. The only change I had to make was to change the “https:\\” reference to “http:\\” (note deleted “s”).
As noted in the post, this works well for accessing your company’s e-mail with Entourage on a Mac, but really there seems to be no reason why this wouldn’t work on other e-mail clients, including those found on mobile phones.
I’m a little surprised to see that the author drops the “s” out of the address, meaning that the Secure Socket Layer (SSL) call has been dropped, but if it works, then that’s great.