Basically,
Right now, I build the email headers myself and use the php mail function to send the mail. If you have 200 emails to go out.. this can really really affect your server.
Vbulletin has a function called vbmail, that sends out the emails in batches.
The problem I'm having, is when I use vbmail, I cannot get the headers to build using the $foruminfo and $threadinfo variables.
I dreamed in code last night and had a few more ideas to try LOL!