AppleScript to Intercept Outgoing Mail from Mail.app

Question

Hi,

I'm trying to write an AppleScript to intercept outgoing mail messages from Mail.app.

I want to check that, if I'm sending a message to work colleagues (who're in my Work address book group) that I'm using my work account. Otherwise, my threads get all messed up when people reply to the wrong address, etc.

There are two parts to my question:

1. How can I trigger a script to look at an outgoing email?
2. How can I prevent a message from being sent?

So far, I've tried using outbox rules. These allow me catch the right emails, but I'm not sure how I can then prevent an errant email from sending.

Any assistance you could offer would be much appreciated.

Cheers, Ross