Hey there!
I’d look here first, although I know you’ve already tried at least one of these steps:
Rebuild the Spotlight Index on your Mac:
https://support.apple.com/en-us/HT201716
If that doesn’t do the job, I’ve come across this a number times. Different OS’ call for certain fixes if the OS doesn’t do the job itself, this should do it:
Quit Mail.
Find and delete this file:
Open Finder.
While holding the Option key, click on Go on the top menu and click Library, (You have to be holding Option to see it).
In the ~/Library, open MetaData/CoreSpotlight, delete the contents of CoreSpotlight, (Not the folder itself).
Restart your Mac, wait a while and see if it works.
If it doesn’t, open Terminal and press Return after each of these three commands:
Finder > Go > Utilities > Terminal:
sudo mdutil -i off /
sudo mdutil -E /
sudo mdutil -i on /
Be sure to enter them correctly, and it’s always a good idea to keep a backup, although I’ve never had an issue, everyone has their own setup.
Give it a while, even up to a day, then test out Mail, hope that helps!