On RooJs.com there's a recent post from Alan Knowles looking at how to spam in PHP. No, nothing malicious - it's more about scalability in sending emails from PHP applications.
The reason this has been relivant in the last two weeks is two fold, first off, my slow and sometimes painfull Pman.Core and Pman.Base). It seemed like an ideal time to write some generic code that can solve both issues.
He mentions the usual method of generating numerous emails and sending them to a remote SMTP server, but points out that there's a better way. You can take advantage of queuing and batch sending techniques and, the way he decided to do it for mtrack, using queue tables and a backend runner (a cron job) that uses proc_open to send out multiple emails at once. You can see the code for it here.