Sending Mail on Forge

I have a Laravel website that works fine on Forge. I have just added a contact form to the site and tested it locally. The form works and I receive the email.

So, I pushed the changes to the server and now when I try and send the contact form I get a 500 server error.

Does anyone know if there is something I need to do to get it to work on the server?

I'm using Mailgun.

Thanks.