Webhook and inbound route URLs should be set up to accept, at a minimum, POST requests.
When you provide the URL where you want Mandrill to POST the data for events, we'll do a quick check that the URL exists by using a HEAD request (not POST). If the URL doesn't exist or returns something other than a 200 HTTP response to the HEAD request, Mandrill will fallback and attempt a POST request.
The POST will be the same type of POST as a Mandrill webhook, except that the
mandrill_events parameter will be an empty array, and the POST will be signed with a generic key (with the value 'test-webhook').