Create setting SwiftMailer in Slim Framework.

app.php

$app->mailer = function () {
    $transport = Swift_SmtpTransport::newInstance('smtp.gmail.com', '465', 'ssl')
                ->setUsername('example@gmail.com')
                ->setPassword('password');
                
    $mailer = Swift_Mailer::newInstance($transport);
    
    return $mailer;
};

$mailer = $app->mailer;