Finder / spotlight / mail search not working
I can also confirm that if you run out of disk space Apple Mail spotlight can become unusable.  My solution is slightly left field but has the advantage of being utterly reliable - rather than trying to rebuild OSX spotlight or force mail to reindex mail, I instead archive mail into EagleFiler (https://c-command.com/eaglefiler/).  EagleFiler maintains its own separate search index so is unaffected by system spotlight issues which I find difficult and time consuming to resolve.   Double clicking a mail message in EagleFiler will open it (and any attachments) in Mail.