framework/mail/BaseMessage.php
public function send(MailerInterface $mailer = null)
    {
        if ($mailer === null && $this->mailer === null) {
            $mailer = Yii::$app->getMailer();
        } elseif ($mailer === null) {
            $mailer = $this->mailer;
        }

        return $mailer->send($this);
    }
public function send(MailerInterface $mailer = null)
    {
        if ($mailer === null && $this->mailer === null) {
            $mailer = Yii::$app->getMailer();
        } elseif ($mailer === null) {
            $mailer = $this->mailer;
        }

        return $mailer->send($this);
    }
Sends this email message.
@param MailerInterface $mailer the mailer that should be used to send this message.
If no mailer is given it will first check if [[mailer]] is set and if not,
the "mailer" application component will be used instead.
@return bool whether this message is sent successfully.