Most of us have customers who, for one reason or another, like to click-around in the SBS 2003 interface. Hopefully they refrain from making changes without consulting a professional, but hey – if it’s their system, they can do what they want (unless they have managed services agreement that specifies they wont click-around at random).
So I had a customer call that was very concerned about the MTA service being disabled in SBS 2003. I'm not really sure how they arrived at their concern, nor were they interested in explaining what prompted the conversation, but they wanted to know why the service was disabled.
Under SBS2003, the "Microsoft Exchange MTA stacks" service doesn't do anything by default. So it's disalbed. Now, it's there to handle mail transport between Exchange 5.5 servers or third-party connectors - so, GroupWise, Notes, Faxmaker 10, etc. So unless you have a very specific need, you can leave it disabled. In an Exchange 2000 or Exchange 2003 native-mode environment, you can disable the MTA stacks service, and there's a KB article that details this.
Long-story short, on SBS 2003 the MTA stacks serivce is disabled by default. You can leave it that way unless you have a specific need.