Apple Mail Search Mojave 10.14.6 after security update 2020-003 defective

I tried several things besides Mailbox>Restore and the "Reindex trick" (deleting the 3 "envelope..." files from the appropriate folder in ~/Library...), which all did not help. Mail.app did not find the very mails that I saw in the list of the mails of Mail.app and that were found using Spotlight directly (cmd-space). All smart mailboxes showed "0 E-Mails" on top of that.

Although Indexing was on - I removed the only volume of my iMac by dragging it into the "Privacy" Tab of the spotlight settings, and after a few minutes out of there again, which triggered indexing the whole volume - there was only one single command in terminal that finally turned everything back to working perfectly again:

sudo mdutil -a -i on

I could watch the indexing progress in the spotlight search window (progress bar).

After it finished, I started Mail.app again, and all smart mailboxes immediately have shown their proper number of E-Mails. Search works like charm again now from within Mail.app. I don't understand why only this command helped, which basically turns on indexing on all volumes, and why the other mail client I had installed in the meantime did show search results.

Hi Flexibug,

I understand you're having issues searching for Mail, and I want to help.

I see you tried some steps to get your mailboxes working again. Did you try to use the option to rebuild your mailboxes?

If you can't find relevant messages during searches, it's a good idea to use the rebuild option:

Rebuild mailboxes in Mail on Mac - Apple Support

Thanks for using the Apple Support Communities.

Hi sterling r,

Thank you for your offer to help, I am a bit stuck here. Yes, I did "rebuild" the mailboxes (several times already before) following the given instructions.

It seems like the problem is bound to the mail.app on this particular iMac, not to Spotlight, though. Calling Spotlight directly (cmd-space) I do get (a limited number) of the expected e-mails when searching e.g. for "Apple" - in contrast to searching for the same expression within the mail.app, which returns an empty list, a.k.a "nothing". I was able to install "Spark" (another mail client) and all works fine there. However, I'd rather have that original apple mail client back to working again.

Are there any settings/parameters/options that could be checked that might prevent the mail.app to use the indexes properly? Like delete a potentially corrupted .plist file and have it rebuilt automagically on a next start of mail.app?

When I try using some "search suggestions" dropping from the search field in the mail.app by clicking on any, I get a blank result list. The Apple Mail client basically finds nothing at all. Each and every one of the "smart mailboxes" are empty when I click on any, like "Today" or "Unread", although I can see those mails in the list of e-mails in the inbox. Very strange behavior...