I have a PHP app I have written for clients to create HTML newsletters and send them out to emails in a database. Mailing is done via the mail() command (I know, crappy), fr
For this kind of volume, you'd be much better off going with an external service, such as
If you try to do this yourself, you'll more than likely end up getting flagged as a spammer. In addition, you need to be careful of managing opt-out lists and such (as per CAN-SPAM).
These services will usually have an API, so it should be relatively simple to integrate with your current codebase without any trouble.
Using a such a service will mean less work in the long run, better compliance, less trouble with getting marked as spam, and the likes.
(Information compiled from a related Something Awful thread.)
Check out SwiftMailer - it is much better than PHPMailer and is actively being developed (PHP Mailer seems sporadic at best)