Autonomous Mass Mail List System (Broadcast Email)

Hey Everyone, I'm working a project for one of the company's websites. they have a subscription mailing list that the user signs up their email address to receies periodical emailings. The email addresses are stored in a flat txt file. 1 email per line. I could also convert it to a coma delimited txt file if it would be easier. So, they've had problem with people who no longer want to receive emails, and one's that bounce for whatever reason, generally a bad email address. What I am trying to find out if who has some information to set up a system where we will send out our emails to the subscribers with our mailing list manager. when i customer respondes with a certain command in the body or subject (like unsubscribe) or if the mail bounces, I'd like it to search out the email address and delete it. Now, I know there's lists like majordomo that do something similar. But I'm looking for something a little more straightforward. I also know this may be more along the lines of shell scripting, but I thought some of you ingenious people may have already done something similar or have some ideas on where to start. thanks.