6 - PHPMailer
PHPMailer is a code library to send (transport) emails safely and easily via PHP code from a web server (MUA to the MSA server).
Sending emails directly by PHP code requires a high-level familiarity to SMTP standard protocol (RFC 821, RFC 2821 and RFC 5321) and related issues (such as Carriage return) and vulnerabilities about Email injection for spamming. From 2001 PHPMailer is one of the popular solutions for these matters on PHP.
You can see examples of use here