To enable Spotlight indexing of email in Postbox, go to Preferences | General and check "Allow Spotlight to to search messages". To immediately get all your mail into Spotlight, do Tools | Indexing | Index All Messages...
Advanced Settings for the Power User
By default, Spotlight will not index any messages in your Trash folders, Junk folders, or messages marked as Junk.
To enable Spotlight indexing of messages in your Trash folders, use the Config Editor and set the hidden pref mail.spotlight.ignore_trash to false.
To enable Spotlight indexing of messages in your Junk folders and message marked as Junk, use the Config Editor and set the hidden pref mail.spotlight.ignore_junk to false.
Unchecking "Allow Spotlight to to search messages" can temporarily cause Postbox to "beach ball" (hang) while Postbox removes all messages from Spotlight.
Spotlight integration does not work with multiple Postbox profiles.
The Spotlight files are located at: ~/Library/Caches/Metadata/Postbox/