For me this was working until recently. Either the latest OS update or the iCloud+ additions have caused it to stop working. I have rebuilt all mailboxes, cleared the cache, and checked the checkbox in the spotlight settings.
Now, I guess I have to rebuild again. My question is what has caused it to suddenly change when it has been working perfectly for years? On another note the latest Safari update seems to be awful as well every time I open it within a few minutes the fan goes full speed like the laptop is going to take off and it becomes unresponsive especially when using the inspector. I have switched back to Chrome and almost ready to transfer back to a Windows laptop, since it seems Apple is caring less and less about user experience. Not really the value and quality I have been use to.