A message represents the settings and content of an email, such as the sender, recipient,
subject, body, etc.
Messages are sent by a [[\yii\mail\MailerInterface|mailer]], like the following,
~~~
Yii::$app->mailer->compose()
->setFrom('from@domain.com')
->setTo($form->email)
->setSubject($form->subject)
->setTextBody('Plain text content')
->setHtmlBody('HTML content')
->send();
~~~