Laravel 5.1 GuzzleHttp Client error: 404 when using mailgun

I'm using the Mailgun service when sending mail with Laravel. However, i've set this up today and it's just stopped working. I have entered all the correct info in .env, config/services.php and config/mail.php. However i'm still getting the below error:

ClientException in Middleware.php line 69: 
Client error: 404

It looks like the domain is not getting passed through somehow, even though in my config/services.php file I have:

'mailgun' => [
    'domain' => env('mydomain.com'),
    'secret' => env('<my-mailgun-key>'),
],

I have hidden the above credentials for safety, but in my real application they are the proper values.

Please help.

via Chebli Mohamed