Craft 3 multi-env email settings

app.php

<?php
/**
 * Yii Application Config
 *
 * Edit this file at your own risk!
 *
 * The array returned by this file will get merged with
 * vendor/craftcms/cms/src/config/app/main.php and [web|console].php, when
 * Craft's bootstrap script is defining the configuration for the entire
 * application.
 *
 * You can define custom modules and system components, and even override the
 * built-in system components.
 */

return [
    'modules' => [
        'my-module' => \modules\Module::class,
    ],
    //'bootstrap' => ['my-module'],

    '*' => [
    ],

    'dev' => [
        'components' => [
            'mailer' => function() {
                // Get the stored email settings
                $settings = Craft::$app->systemSettings->getEmailSettings();

                // Override the transport adapter class
                $settings->transportType = \craft\mail\transportadapters\Smtp::class;

                // Override the transport adapter settings
                $settings->transportSettings = [
                    'host' => 'smtp.mailtrap.io',
                    'port' => '2525',
                    'useAuthentication' => true,
                    'username' => 'USERNAME',
                    'password' => 'PASSWORD'
                ];

                return craft\helpers\MailerHelper::createMailer($settings);
            }
        ]
    ],

    'staging' => [
        'components' => [
            'mailer' => function() {
                // Get the stored email settings
                $settings = Craft::$app->systemSettings->getEmailSettings();

                // Override the transport adapter class
                $settings->transportType = craft\mailgun\MailgunAdapter::class;

                // Override the transport adapter settings
                $settings->transportSettings = [
                    'domain' => 'YYYYYYYYYYYY',
                    'apiKey' => 'key-YYYYYYYYYYYY',
                ];

                return craft\helpers\MailerHelper::createMailer($settings);
            }
        ]
    ],

    'production' => [
        'components' => [
            'mailer' => function() {
                // Get the stored email settings
                $settings = Craft::$app->systemSettings->getEmailSettings();

                // Override the transport adapter class
                $settings->transportType = craft\mailgun\MailgunAdapter::class;

                // Override the transport adapter settings
                $settings->transportSettings = [
                    'domain' => 'ZZZZZZZZZZZZ',
                    'apiKey' => 'key-ZZZZZZZZZZZZ',
                ];

                return craft\helpers\MailerHelper::createMailer($settings);
            }
        ]
    ],
];