cakephp/tests/test_app/TestApp/Config/TestEmailStaticConfig.php

<?php

namespace TestApp\Config;

use Cake\Core\StaticConfigTrait;

class TestEmailStaticConfig
{
    use StaticConfigTrait;

    /**
     * Email driver class map.
     *
     * @var array
     */
    protected static $_dsnClassMap = [
        'debug' => 'Cake\Mailer\Transport\DebugTransport',
        'mail' => 'Cake\Mailer\Transport\MailTransport',
        'smtp' => 'Cake\Mailer\Transport\SmtpTransport',
    ];
}