helper.php

public static function emailTemplate($viewName = '', $toEmail = '', $subject = '', $dataToSend = array())
{  
    $from = \Yii::$app->params['FROM_EMAIL'];
    $bcc_email = \Yii::$app->params['BCC_EMAIL'];

    \Yii::$app->mailer->compose($viewName, $dataToSend)
                       ->setFrom($from)
                       ->setTo($toEmail)
                       ->setBcc($bcc_email)
                       ->setSubject($subject)
                       ->send();
}

main-local.php

<?php
return [
  'mailer' => [
    'class' => 'yii\swiftmailer\Mailer',
    'viewPath' => '@common/mail',

        'transport' => [
            'class' => 'Swift_SmtpTransport',
            'host' => 'smtp.gmail.com',  // e.g. smtp.mandrillapp.com or smtp.gmail.com
            'username' => '[email protected]', // my email id
            'password' => 'xxxxxxxxx', // my password
            'port' => '465', // Port 25 is a very common port too
            'encryption' => 'ssl', // It is often used, check your provider or mail server specs
        ],
    ],
]
?>