Apple Mail will not search Entire Message

If you find that Apple Mail will not allow searching emails by message contents because the menu item is grey and not selectable, it is because Spotlight needs to reindex the hard drive volume.

First make sure that your drive is not in the Privacy option in the Spotlight preferences.

If that does not correct the problem, you need to open terminal and enter the following commands:

sudo mdutil -i off /Volumes/HardDriveName

and then

sudo mdutil -i off /Volumes/HardDriveName