environments/dev/common/config/main-local.php

<?php
    return [
        'components' => [
            'db'     => '',
            'mailer' => [
                'class'            => 'yii\swiftmailer\Mailer',
                'viewPath'         => '@common/mail',
                // send all mails to a file by default. You have to set
                // 'useFileTransport' to false and configure a transport
                // for the mailer to send real emails.
                'useFileTransport' => true,
            ],
        ],
    ];