How to send email from Craft 3 custom plugin?

Question

I have created custom plugin in Craft 3 and want send email to receiver or site email when user submits a form.

Answer 1

Edit: After the migration to Craft 3.1.x you should rather use this Code

/**
 * @param string                            $html
 * @param string                            $subject
 * @param array|string|\craft\elements\User $mail
 *
 * @return bool
 */
public function sendMail(string $html, string $subject, $mail): bool
{
    return Craft::$app
        ->getMailer()
        ->compose()
        ->setTo($mail)
        ->setSubject($subject)
        ->setHtmlBody($html)
        ->send();
}

Edit: this is my old answer, valid for Craft 3.0.x

This is my code to send emails in Craft 3. Let me know if you have any questions.

use craft\mail\Message;

/**
 * @param $html
 * @param $subject
 * @param null $mail
 * @param array $attachments
 * @return bool
 */
private function sendMail($html, $subject, $mail = null, array $attachments = array()): bool
{
    $settings = Craft::$app->systemSettings->getSettings('email');
    $message = new Message();

    $message->setFrom([$settings['fromEmail'] => $settings['fromName']]);
    $message->setTo($mail);
    $message->setSubject($subject);
    $message->setHtmlBody($html);
    if (!empty($attachments) && \is_array($attachments)) {

        foreach ($attachments as $fileId) {
            if ($file = Craft::$app->assets->getAssetById((int)$fileId)) {
                $message->attach($this->getFolderPath() . '/' . $file->filename, array(
                    'fileName' => $file->title . '.' . $file->getExtension()
                ));
            }
        }
    }

    return Craft::$app->mailer->send($message);
}