My first question is how many people are you sending to? If it's less than a couple hundred, you might find it cheaper to manually do it, like in Outlook or something. If more though, you'll want an automated web-based system to control it.
One issue you need to worry about is if your customers consider it junk mail or not. You don't want to end up on a blacklist because you were reported...
I don't know of any automated systems off the top of my head, but you should do some digging in sourceforge.net and google for any open-source apps to do it.