For God's sake people, Exchange/Outlook was designed to be a LAN application, NOT an Internet application. If you've got to access your e-mail over the Internet and just gotta have MS-based clients, use the POP or IMAP protocols. Even better use the SSL-based versions of those protocols, or use OWA or a VPN.
Using a standard Outlook/Exchange configuration opens you to problems such as the Blaster worm.
Analogy: Tractors. Tractors are designed for driving back and forth across your property, doing heavy chores like plowing, bailing hay, or "spreading" manure. They are NOT designed to be taken out on a four-lane highway to get you from point A to point B. You not only run the risk of getting killed when the engine explodes, you inconvenience every other user of that four-lane and most likely anger local authorities.
Take a close look at how you do business and drop me into the Thoroughly Disgusted category.