Help Me Out Here

I’m using Apple Mail with two email accounts, one for work and one for my personal life. Currently, the default account is my personal account. So, I end up accidentally emailing work people from my personal address instead of from the work address. That isn’t the end of the world, but it’s annoying.

What I need is some sort of hack or plug-in for Apple Mail where I can essentially say: “When I email John Smith use this account, but when I email Sally Jones use this other account.”

Does anyone know of anything that does this?