Allow per-message webhooks
It would be really helpful if there were a way to specify webhooks at the message level as well as the domain level. (Perhaps the message-level webhook, if present, would override the domain-level webhook.)
I send some emails from one server, and some emails from another. It would be really nice to be able to have the postbacks go to the right place. I can emulate the functionality by querying Mailgun about each message at intervals, but that's awfully inelegant.