STCav
Dear all,
This looks to me like a corruption of the Mail indexes, which I've come across a couple of times previously. I've found a solution that worked for me.
With Mail closed, in the Home | Library, go to Mail | V2 | MailData. Delete the three files with names starting with "Envelope":
- "Envelope Index"
- "Envelope Index-wal" and
- "Envelope Index-shm"
(Drag them to the desktop if you want to be cautious!) Open Mail again and it goes through an importing and indexing process, forcing a rebuild of the index from scratch, which resolved the problems that I experienced, which included faulty "read" count, missing folders, invisible messages, etc.
This is the best answer of all.
The nice thing about having a mac is that lots of stuff can be fixed like this sometimes.
Sometimes your computer gets confused and then you need to tell it to sort things out again.
But it did not help me.
There's something bigger at hand.