A recent message has completely disappeared in Mail

I had a number of messages in my inbox, and today one of them, that I was planning to respond to, wasn't there anymore.

I have tried to rebuild the mailbox (IMAP) and have looked at the inbox directly using my provider's webmail in a browser. I have searched in the trash and have searched everywhere else, either using mail's search or InfoClick (a utility from Nisus Software).

I know that I can send a message to the trash by accident (it's actually much too easy to do that) but even then, I should be able to find it. I also checked that it wasn't filtered.

I'm using only two plugins: Mail Act-on and Antidote grammar checker. I've been using them for years. Oh, and I'm on Sierra (10.12.6)

If I can't trust Mail anymore, it's going to be tough.

Any ideas? Has this happened to other people?

EllenH