Subject does most of the work here, but I'll expand. [edit] Maybe if I had just RTFM'd harder I would've found my answers... I think this will help, but leaving the post up in case anybody else has ...

With the 2007 version, you can now set up Exchange as an edge server with all the security of Sendmail Nobody with an ounce of security sense would plug a Web connection directly to an e-mail server ...

We have people that travel and I've been annoyed a lot with some hotels using ISPs that block SMTP port 25 traffic. The solution is having to talk the users through changing the default SMTP server ...