Fixing smart mailboxes showing the wrong messages / erasing spotlight indexes on macOS Catalina

macOS Catalina introduced some new changes that means many of the articles you'll find about resetting spotlight to fix smart mailboxes not working properly are wrong.

Firstly you can use this command to see the status of the indexes on all volumes:

mdutil -s -a -v

and this command will reset the indexes for your root storage and should kick off a reindex (if you check with Activity Monitor you should see mds_stores uses lots of CPU to generate the new index):

mdutil -E /System/Volumes/Data

(Most blogs suggest using the path of just /, which won't help on Catalina as that partition now only contains system files and none of the user data.)

Some people suggested the -X flag is necessary in some cases, but I'm not convinced.