Using PHPMailer's own autoloaderNot so long ago, PHPMailer changed the way that it loaded classes so that it was more compatible with composer, many frameworks, and the PHP PSR-0 autoloading standard. Note that because we support PHP back to version 5.0, we cannot support the more recent PSR-4 standard, nor can we use namespaces. Previously, PHPMailer loaded the SMTP class explicitly, and this causes problems if you want to provide your own implementation. You may have seen old scripts doing this:
require 'class.phpmailer.php';
If you do only that, SMTP sending will fail with a Class 'SMTP' not found error. You need to either explicitly include the class.smtp.php file (read the README for info on which files you need), or use the recommended approaches of using composer or the supplied autoloader, like this:
require 'PHPMailerAutoload.php';