Mail indexing stopped two weeks ago

My problem is similar to a million problems reported here, mostly without a solution. But because I've narrowed it down more than usual, perhaps someone can help me solve it.

Mail.app stopped indexing messages on July 8. My smart mailboxes list no message later than that date. I have reindexed my Mac multiple times. I have turned my accounts on and off. I have not deleted my Envelope folders because I am not at home, with slow Internet connections, and I know that if I do this, it would be days and days of re-downloading before Mail.app works at all again.

But I have narrowed down the problem to a message or messages located in an Exchange Inbox from a particular email address (airbnb.com) on that particular day. This is because all my Smart mailboxes that include this Inbox show a badge for two unread messages that disappears if I narrow the mailbox down to any other dates and email addresses, and reapears if I create a Smart mailbox specifically for that address and date.

But the relevant message is nowhere to be found. Nothing to delete and nothing obvious to fix.

Any suggestions? Is there some non-Envelope file in any of the folders related to Mail that I could delete to end this problem?