On the php|architect site there's a recent tutorial from Cal Evans about building a transactional email system with the help of a simple library and the SES system from Amazon.
I love APIs. A well-defined API can make short work of a complex problem. It's even more fun when you find a cool API with a great wrapper to make it simple to use. That's what I found when I started playing with Amazon's Simple Email Service; a tool that was easy to work with, solved a problem I needed solved, and had a simple to use PHP wrapper.
Based on the SES library by Dan Meyers, he includes the code to pull in a simple email template and populate it with the values you want, log in to the Amazon SES service and send the email via the remote service.