send mail to mailing list using mail() or smtp?

I would appreciate some feedback. I am writing a script to fire off emails in HTML or text format to everyone in a mailing list. It works no problem - but I am concerned with how many emails it can deal with over time. The script has no timeout setting, I am not sure if it needs one. It fires off 100 emails, pauses for 1 minute using sleep(60), and then fires off another 100. It sends the mail using mail(). I did my first send the other day to 930 addresses and it worked fine. My question - is this going to work for a few thousand emails? What about 100,000? Would it be better to send the mail directly to the smtp server? Can you foresee any problems? I look forward to your reply... Regards, Matthew Delmarter