Bulk mail with Net/SMTP

Hello, I have been using the mail() function to send out announcements for a newsletter. This has had a few issues, now that the list is at 400 mail only goes out to the 1st member on the list. No errors, I assume there is a communication issue with the local smtp server here. So I am now using Net/SMTP, small tests work great. Is it acceptable to add over 400 folks with rcptTo, 10 seems fine. Is there a more robust/scalable approach with PHP? Thanks,