That is very interesting and might be what's going on. Yes, I've been using Clear mail MX as backup.
I read about this over the weekend and changing some settings in either Spamassassin or Postfix but I can't remember which. I flew over my head as I didn't even thing about the Clear MX. It was along the lines of making sure that email only came in via the lower MX records. But that screws up if the server goes down for a while. Then all the email go into the black hole.
I'll collect the maillog info this evening and post.