In mail.app can one set the default account that is used to send mails to a specific person In mail I use two email accounts, a gmail for private use and an email adress that was allocated to me for use when mailing to teachers and staff. 

I would like to set mail.app to automatically use my private email adress when I'm mailing to my friends and my university's email adress when mailing to my teachers etc.

I want this because now I tend to forget to change this account, using the gmail when mailing to teachers and using the universities account when mailing to friends, which can cause some problems when they reply, having emails end up in the wrong mailboxes and so forth.

Can it be done? 

Currently I'm on Snow Leopard, but I will be moving to Lion soon.
You actually can set a rule to move the received message to different folders according to the sender, either your friends or your university's teach.

In terms of the automatically sending message to different receipt with different account, I can't figure it out, but there is a manual way. In the mail.app Accounts setting, leave the "User only this server" un-selected.  So that, when you compose a message, there will be a SMTP server selection drop down list allow you to manually select which SMTP server you are going to use.