Uses new MailgunEmail that is much more compatible with PHPUnit and DebugKit. You can use MailgunEmail or the original Email and add the additional headers which can be found at https://documentation.mailgun.com/en/latest/user_manual.html#sending-via-smtp
The MailgunEmail class allows you to quickly add the required headers instead of doing it at the Transport level which caused major issues with PHPUnit and DebugKit.