PHPMailer


PHPMailer is a code library to send emails safely and easily via PHP code from a web server.
Sending emails directly by PHP code requires a high-level familiarity to SMTP standard protocol and related issues and vulnerabilities about Email injection for spamming. From 2001 PHPMailer is one of the popular solutions for these matters on PHP.

Features

Partial list of features:
Worx International Inc claimed in 2009 that: "PHPMailer continues to be the world's most popular transport class, with an estimated 9 million users worldwide. Downloads continue at a significant pace daily.".
PHPMailer sends an SMTP extension command with the sent emails as below which can be used to get the usage report of PHPMailer on the ESPs :
X-Mailer: PHPMailer 5.2.13

This SMTP header may differ for different versions of PHPMailer, and can also be suppressed or modified by PHP code.