Total failure of Search of Apple Mail

Running OS X El Capitan 10.11.6

on MacBook Pro Retina 15inch

If I take a random email’s subject line and paste it entirely into Apple Mail search - the result is zero

If I take exact same subject line and paste into the Mac's Spotlight search - the result is again zero

If I take a file name at random from my hard drive and paste into Spotlight search - the result is 100% perfect

If I look into any of my “smart mailboxes” (which I use a lot) they are all EMPTY.

It seems to me that anything to do with search relative to Mail is buggered.

Please anyone who can help for sure?