Sending out Personalized Mass Messages (small scale)
I've used constant contact before to send out mass marketing communications. And while this is a great service, spending $15/month to send out emails may be too much for your small business. If you'd like to try it out though, they are offering a 60 day free trial.
Luckily, there is a free way to do this on a small scale for free with gmail. It seems that you can get through 100 emails a day without your email being locked for a day. Instructions for this can be found here. If you'd prefer to use Outlook, instructions on how to mail merge are here.