Apple Mail: React on an outgoing message before it is sent, possible?

Hi all,

sorry, if this has already been answered somewhere or if it is obvious (I’m an Applescript-newbie).

I just wanted to know if it is possible to make an Appelscript that is invoked when mail starts to send an email. More specifically, I’d like to test a mail that is going to be sent on certain criterias and maybe change some of them before the mail is actually sent away.