I think PHP works with sendmail, and probably also most major MTAs.
From the PHP manual:
For the Mail functions to be available, PHP must have access to the sendmail binary on your system during compile time. If you use another mail program, such as qmail or postfix, be sure to use the appropriate sendmail wrappers that come with them. PHP will first look for sendmail in your PATH, and then in the following: /usr/bin:/usr/sbin:/usr/etc:/etc:/usr/ucblib:/usr/lib. It's highly recommended to have sendmail available from your PATH. Also, the user that compiled PHP must have permission to access the sendmail binary.
Something called EZComponents might help, it's a library of PHP components. You only need one of them: the mail component. I believe it contains lots of options and allows you to open sockets directly and send mails - without the sendmail wrapper. In other words, it contains an implementation of the SMTP protocol. Probably the POP protocol too.
I want to install an MTA on my server to receive and send mails, and provide email to some domains I host. But I have yet to find one that is easy to install and configure... If you find any, let me know!