email validation does not accept POST requests
It appears that your API endpoint for ttps://api.mailgun.net/v4/address/validate is not correctly accepting valid requests.
That endpoint accepts GET requests, but not POST requests. This is incorrect because the endpoint relies on a form-url-encoded body, which should not be send in a GET request as per the HTTP spec. Instead a form-url-encoded body should be sent during a POST (or PUT) request.
I suppose it's fine to allow GET requests to work (even if they are technically outside the HTTP spec), but a POST request should definitely work.
I spent a few hours this morning ******* my head against this problem and finally figured it out.