What I am planning to do is build a back-end mail list manager in php.It will query the mysql data base for specific customer profiles and then send an email to that group of people. These mailings could range from a few hundred to a few thousand.
Once I have the query in a array it would then be put in a loop and send each customer using the php mail command.
The only thing is that PHP is not the best way to write bulk mail programs. For what you have, it'll be fine, just keep in mind that It'll be better to mail at times when the system is less busy. If the lists get really large you may want to think about using perl or python or something (not my area ;))