di.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
/**
 * di
 *
 * @copyright Copyright © 2017 Staempfli AG. All rights reserved.
 * @author    juan.alonso@staempfli.com
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Framework\Mail\TransportInterfaceFactory">
        <plugin name="staempfli_mailcatcher_handle_mail_transport" type="Staempfli\MailCatcher\Plugin\HandleMailTransportPlugin" sortOrder="-99"/>
    </type>
    <preference for="Magento\Framework\Mail\MessageInterface" type="Staempfli\MailCatcher\Mail\Message"/>
    <!-- A preference for "Magento\Framework\Mail\Message" it is also needed because stupid Magento is loading
        this class directly and not the Interface at Magento\Framework\Mail\Template\TransportBuilder::reset()
        See PR: https://github.com/magento/magento2/pull/11152 -->
    <preference for="Magento\Framework\Mail\Message" type="Staempfli\MailCatcher\Mail\Message"/>
</config>