Apologies if this is answered elsewhere—couldn’t find an exact match.
This message appears in the inbox. On my Mac, I go to Mail menubar / Message / SpamSieve - Train as Spam. The message disappears. A moment later, it pops back up in the inbox. I repeat, it disappears, then reappears. It’s bounced back and forth about 10 times, but won’t stay gone.
I’ve noticed on subsequent disappearances that, although it vanishes from the inbox, it never shows in the actual Junk folder.