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), from a CRON job with proper pauses, etc. Up to now, all clients have had limited lists (the maximum was 8000 mails, sent in two hours). The server is my own - no restrictions there. The software handles bounced mails, opt out options, etc.
A new client wants to send out 100,000 mails a month in 25K weekly batches and I want to replace the mail() function. Any suggestions? Would the PHPMailer class do the trick?