UaMV I think we're missing documentation for that particular function.
You do need to install Guzzle manually in order to use the Mailgun driver.
Because many extensions include Guzzle, including the OAuth login extensions, most people have it by default. But if you didn't install fof/oauth when going from beta 13 to 14, or if you removed that extension manually, you might not have it.
You can install Guzzle using the following command in the Flarum folder:
composer require guzzlehttp/guzzle