Can Mail warn when emailing certain addresses?

I have my own personal mail account (a gmail account) and my academic email account (a university account) as different profiles on my Mail application in OS X Snow Leopard. I sometimes forget to switch the outgoing account to my university account when emailing my students and it will send from my gmail account instead. This is not a huge deal but I would prefer to have my students never get mail from my gmail account.

Is there a way to have Mail warn you when sending to a certain address (or an address that has a certain keyward in it?)

Yes, you can have Mail put certain addresses in the to field in black and all others in red.
I use this to have Mail flag email that's going outside my organization.

Do this by going to Preferences > Composing.
Then under the "Addressing" header check "Mark addresses not ending with [ ]." Inside the box put ".nameofyourschool.edu".
Then every email address destined within your school will be black, and all others will be red.
Just the thing to ensure FERPA compliance.