Module isn't overriding default Mail Transport in Magento 2.2

Question

I started developing Magento 2.2 modules about 3 months ago. I did a very simple SMTP module by following what some open source projects did. I could use a free plugin but the real goal here is to learn Magento 2. My module is hosted on a private GitHub repository and gets updated by running composer upgrade on the magento 2 server. I can see the custom fields I created in the backend but whenever magento send a message an error is thrown. At first I thought the problem was with my module but it seems like my Transport class isn't even called. Here is my main module's files.

composer.json

{
    "name": "mymodule/module-smtp",
    "description": "SMTP Extension for Magento 2 to help the configuration of the SMTP (Simple Mail Transfer Protocol) server.",
    "type": "magento2-module",
    "version": "1.1.0",
    "require": {
        "php": "7.0.2|7.0.4|~7.0.6|~7.1.0",
        "magento/framework": "101.0.*",
        "magento/module-config": "101.0.*",
        "magento/module-store": "100.2.*"
    },
    "license": [
        "proprietary"
    ],
    "minimum-stability": "stable",
    "autoload": {
        "files": [
            "registration.php"
        ],
        "psr-4": {
            "MyModule\\Smtp\\": ""
        }
    }
}

registration.php

<?php

\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'MyModule_Smtp',
    __DIR__
);

etc/di.xml

<?xml version="1.0" encoding="utf-8"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\Framework\Mail\Transport" type="MyModule\Smtp\Plugin\Magento\Framework\Mail\Transport" />
</config>

etc/module.xml

<?xml version="1.0" encoding="utf-8"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="MyModule_Smtp" setup_version="1.0.0">
        <sequence>
            <module name="Magento_Backend"/>
        </sequence>
    </module>
</config>

Plugin/Magento/Framework/Mail/Transport.php

<?php
namespace MyModule\Smtp\Plugin\Magento\Framework\Mail;

class Transport implements \Magento\Framework\Mail\TransportInterface
{
    /**
     * @var \Magento\Framework\Mail\MessageInterface
     */
    protected $_message;

    /**
     * @var \MyModule\Smtp\Helper\Data
     */
    private $_dataHelper;

    /**
     * @var \Magento\Framework\Encryption\EncryptorInterface
     */
    private $_encryptor;

    /**
     * @param \Magento\Framework\Mail\MessageInterface $message
     * @param \MyModule\Smtp\Helper\Data $dataHelper
     * @param \Magento\Framework\Encryption\EncryptorInterface $encryptor
     */
    public function __construct(
        \Magento\Framework\Mail\MessageInterface $message,
        \MyModule\Smtp\Helper\Data $dataHelper,
        \Magento\Framework\Encryption\EncryptorInterface $encryptor)
    {

        $this->_dataHelper = $dataHelper;
        $this->_encryptor = $encryptor;
        $this->_message = $message;
    }

    /**
     * Send a mail using this transport
     *
     * @return void
     */
    public function sendMessage()
    {
        if ($this->_dataHelper->isConfigurationEnabled()) {
            $this->useSmtp();
        } else {
            $this->useSendMail();
        }
    }

    /**
     * Send mail using a custom SMTP server
     *
     * @return void
     * @throws \Magento\Framework\Exception\MailException
     */
    public function useSmtp()
    {
        $host = $this->_dataHelper->getHost();
        $config = array('port' => $this->_dataHelper->getPort());

        $auth = $this->_dataHelper->getAuthentication();
        if ($auth === 'plain' || $auth === 'login' || $auth === 'cram-md5') {
            $config['auth'] = $auth;
            $config['username'] = $this->_dataHelper->getUsername();
            $config['password'] = $this->_encryptor->decrypt($this->_dataHelper->getpassword());
        }

        $ssl = $this->_dataHelper->getProtocol();
        if ($ssl === 'ssl' || $ssl === 'tls') {
            $config['ssl'] = $ssl;
        }

        $zendMail = new \Zend_Mail_Transport_Smtp($host, $config);

        try {
            $zendMail->send($this->_message);
        } catch (\Exception $e) {
            throw new \Magento\Framework\Exception\MailException(new \Magento\Framework\Phrase($e->getMessage()), $e);
        }
    }

    /**
     * Send mail using PHP mail function
     *
     * @return void
     * @throws \Magento\Framework\Exception\MailException
     */
    public function useSendMail()
    {
        $zendMail = new \Zend_Mail_Transport_Sendmail();

        try {
            $zendMail->send($this->_message);
        } catch (\Exception $e) {
            throw new \Magento\Framework\Exception\MailException(new \Magento\Framework\Phrase($e->getMessage()), $e);
        }
    }

    /**
     * @inheritdoc
     */
    public function getMessage()
    {
        return $this->_message;
    }
}

The only error reported by magento is the following.

system.log

[2017-11-21 23:50:55] main.CRITICAL: Exception message: Unable to send mail.
Trace: #0 /var/www/html/vendor/magento/framework/Interception/Interceptor.php(58): Magento\Email\Model\Transport->sendMessage()
#1 /var/www/html/vendor/magento/framework/Interception/Interceptor.php(138): Magento\Email\Model\Transport\Interceptor->___callParent('sendMessage', Array)
#2 /var/www/html/vendor/magento/module-email/Model/Mail/TransportInterfacePlugin.php(48): Magento\Email\Model\Transport\Interceptor->Magento\Framework\Interception\{closure}()
#3 /var/www/html/vendor/magento/framework/Interception/Interceptor.php(135): Magento\Email\Model\Mail\TransportInterfacePlugin->aroundSendMessage(Object(Magento\Email\Model\Transport\Interceptor), Object(Closure))
#4 /var/www/html/vendor/magento/framework/Interception/Interceptor.php(153): Magento\Email\Model\Transport\Interceptor->Magento\Framework\Interception\{closure}()
#5 /var/www/html/generated/code/Magento/Email/Model/Transport/Interceptor.php(26): Magento\Email\Model\Transport\Interceptor->___callPlugins('sendMessage', Array, NULL)
#6 /var/www/html/vendor/magento/module-user/Model/User.php(414): Magento\Email\Model\Transport\Interceptor->sendMessage()
#7 /var/www/html/vendor/magento/module-user/Controller/Adminhtml/Auth/Forgotpassword.php(73): Magento\User\Model\User->sendPasswordResetConfirmationEmail()
#8 /var/www/html/vendor/magento/framework/App/Action/Action.php(107): Magento\User\Controller\Adminhtml\Auth\Forgotpassword->execute()
#9 /var/www/html/vendor/magento/module-backend/App/AbstractAction.php(229): Magento\Framework\App\Action\Action->dispatch(Object(Magento\Framework\App\Request\Http))
#10 /var/www/html/vendor/magento/framework/Interception/Interceptor.php(58): Magento\Backend\App\AbstractAction->dispatch(Object(Magento\Framework\App\Request\Http))
#11 /var/www/html/vendor/magento/framework/Interception/Interceptor.php(138): Magento\User\Controller\Adminhtml\Auth\Forgotpassword\Interceptor->___callParent('dispatch', Array)
#12 /var/www/html/vendor/magento/module-backend/App/Action/Plugin/Authentication.php(143): Magento\User\Controller\Adminhtml\Auth\Forgotpassword\Interceptor->Magento\Framework\Interception\{closure}(Object(Magento\Framework\App\Request\$
#13 /var/www/html/vendor/magento/framework/Interception/Interceptor.php(135): Magento\Backend\App\Action\Plugin\Authentication->aroundDispatch(Object(Magento\User\Controller\Adminhtml\Auth\Forgotpassword\Interceptor), Object(Closure), O$
#14 /var/www/html/vendor/magento/framework/Interception/Interceptor.php(153): Magento\User\Controller\Adminhtml\Auth\Forgotpassword\Interceptor->Magento\Framework\Interception\{closure}(Object(Magento\Framework\App\Request\Http))
#15 /var/www/html/generated/code/Magento/User/Controller/Adminhtml/Auth/Forgotpassword/Interceptor.php(26): Magento\User\Controller\Adminhtml\Auth\Forgotpassword\Interceptor->___callPlugins('dispatch', Array, NULL)
#16 /var/www/html/vendor/magento/framework/App/FrontController.php(55): Magento\User\Controller\Adminhtml\Auth\Forgotpassword\Interceptor->dispatch(Object(Magento\Framework\App\Request\Http))
#17 /var/www/html/vendor/magento/framework/Interception/Interceptor.php(58): Magento\Framework\App\FrontController->dispatch(Object(Magento\Framework\App\Request\Http))
#18 /var/www/html/vendor/magento/framework/Interception/Interceptor.php(138): Magento\Framework\App\FrontController\Interceptor->___callParent('dispatch', Array)
#19 /var/www/html/vendor/magento/framework/Interception/Interceptor.php(153): Magento\Framework\App\FrontController\Interceptor->Magento\Framework\Interception\{closure}(Object(Magento\Framework\App\Request\Http))
#20 /var/www/html/generated/code/Magento/Framework/App/FrontController/Interceptor.php(26): Magento\Framework\App\FrontController\Interceptor->___callPlugins('dispatch', Array, Array)
#21 /var/www/html/vendor/magento/framework/App/Http.php(135): Magento\Framework\App\FrontController\Interceptor->dispatch(Object(Magento\Framework\App\Request\Http))
#22 /var/www/html/vendor/magento/framework/App/Bootstrap.php(256): Magento\Framework\App\Http->launch()
#23 /var/www/html/index.php(39): Magento\Framework\App\Bootstrap->run(Object(Magento\Framework\App\Http))
#24 {main} [] []

Answer 1

After hours of struggles I decided to install Xdebug on my IDE. If you are not using Xdebug already I recommend you to take a look. I followed my request line by line to finally catch the culprit. When sending an Email magento 2 doesn't use "\Magento\Framework\Mail\Transport" instead it uses "\Magento\Email\Model\Transport".

Instead of overriding both of them in my di.xml I decided to wrap a plugin around "\Magento\Framework\Mail\TransportInterface" since all Transport classes implement this interface.

I believe the reason as why Magento_Mail module uses it's own Transport model instead of relying on the Framework Transport model is because the Magento_Mail module adds more options like the possibility to set a return path.