but offer NO guarantees that it will work.

Make sure that you have a complete BACKUP of the mail folders BEFORE you attempt this procedure.

Quit Mail (not difficult in your case)
Navigate to your HOME folder /Library/Mail and find a file called Envelope Index.
Drag it to your desktop.

Launch Mail

It should not tell you something about reindexing the mail database. In your case, this could take a long time.

FWIW, 106K emails is a little over the top. Have you never thrown any emails away or archived them?

P