I use my MacBook every single day, both at home and at university. Although you can specify more than one outgoing mail server in Apple mail, it is a bit annoying that you have to change the setting every time you change location. Enter MarcoPolo. This is quite a new program, but I use it on a daily basis and am very happy with it. MarcoPolo guesses at which location you currently are and can do a numer of things based on this guess. For me it changes outgoing mail server, default printer and, at university, switches off wifi.
It's free, it works great and, if you need this functionality, I strongly recommend it. Of course it also has builtin Growl support!
Visit MarcoPolo's website.