If you want to write PHP scripts that handle email, there are a few things you need to understand first. This article, the first of two parts, introduces you to these concepts.
The article is an excerpt from the Zend PHP Certification Guide and focuses on some of the basics of handling email with PHP. There's some knowledge that you'll need to have before you get started, but the rest is pretty simple stuff. It's walked through at a good pace, making it easy to follow and a good beginner's resource.
Topics they discuss include working with MX records, preparing your PHP installation to handle emails, and sending an email from your script (including instructions for both Unix and Windows based systems).