Install through Composer:
composer require understeam/yii2-mailgun:^0.2 --prefer-dist
Configure:
...
'mailer' => [
'class' => 'understeam\mailgun\Mailer',
'domain' => 'domain.com',
'apiKey' => 'api-key',
],
...Use:
$message = Yii::$app->mailer->compose('@common/mail/signup', ['user' => $user]);
$message->send();