Another thing to do: if your mailgun email to gmail gets inexplicably marked as spam (as opposed to DKIM failing), try creating a new fresh gmail account and see if email to that gets marked as spam. I was using an old gmail account and its filters were full of cat fur, which was causing stuff to randomly get marked as spam. Spent days playing high-low game deleting parts of the message then adding parts back in trying to figure out which words were triggering it, and it was totally non-deterministic. But once I switched to a fresh gmail account, it worked just fine, and nothing went into the spam folder.