Webhook and inbound route URLs should be set up to accept, at a minimum, POST requests.
When you provide the URL where you want Transactional Email to POST the data for events, we will do a quick check that the URL exists by using a HEAD request (not POST). If the URL does not exist or returns something other than a 200 HTTP response to the HEAD request, Transactional Email will fallback and attempt a POST request.
The POST will be the same type of POST as a Transactional Email 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').