Try to select the inbox then menu>mailbox>rebuild
Thanks - I have tried that for all the folders which seem relevant.
The 'problem' email is in a named Folder, not in the IN folder.
After rebuilding several Folders.including IN, the Activity Viewer still shows Mail attempting to index the thing.
After moving all the messages from the problem folder into several new folders, in an attempt to locate the trouble. the mad indexing is still being attempted.
I could try to ignore it, I suppose, but that might just be storing up trouble for later.