I'm not sure if small business supports the function. But, what your looking for is called a delegate. If you open outlook and go to tools, options you should see the tab for it. You can set permission for another user to read and manage your email. This includes sending emails on your behalf. As mentioned you can show the from field also by opening an email and going to view and select from field. But, I think you have to have delegate permission to send an email as another user.