What's the differences between the Mailer and the Message classes?
They're both valid options.

$message-&gt;send() is a wrapper for Mailer:

https://github.com/yiisoft/yii2/blob/master/framework/mail/BaseMessage.php#L41-L50

It will take an optional Mailer parameter.  If one is not passed it, it will default to the default Yii/Craft::$app->mailer and ultimately just call send() on it.