Mockoon supports partial mocking of an API endpoints by forwarding the requests that does not match a declared route to the URL of your choice.
To activate the proxy mode, open the Environment Settings by clicking on the cog in the upper right corner:
Enable the proxy mode by checking the box and enter the server URL to which you want to forward the calls. A small shield icon should appear on the right of your environment name indicating that the proxy mode has been enabled:
From now on, all routes that have been defined in Mockoon will be intercepted, and any other request not matching any route path will be forwarded to the server URL you entered in the proxy mode setting.
Proxy specific headers can also be added, both to the forwarded request and the response received from the target API.