The good news: HoudahSpot 6.0 can again find Apple Mail message files!
With macOS 10.15 Catalina, Apple moved the indexing of Mail messages files from traditional Spotlight to “Core Spotlight.” The problem with this change is that Core Spotlight does not allow searching by third-party applications like HoudahSpot.
More precisely, applications are only allowed to search data they have submitted themselves. Thus the Mail application can call upon Core Spotlight to find messages. Core Spotlight is of no use to search tools like HoudahSpot. HoudahSpot does not manage or create data and thus never has data to submit to Core Spotlight. Instead, as a HoudahSpot user, you expect to find all kinds of files regardless of which application created the files.
This limitation of Core Spotlight does not only interfere with search tools. Many other utilities, scripts, and productivity tools suffer from the fact that messages, notes, and bookmarks are no longer searchable. Previously, a contact manager like BusyContacts could rely on Spotlight to watch for mail messages and maintain an activity list for each contact.
Apple, of course, grants itself exceptions to the rule. The Spotlight menu item is allowed to search all of Spotlight and Core Spotlight. We asked Apple to grant HoudahSpot the same privilege. No such luck.
We filed bug reports, enhancement requests and asked Apple developer support for help. So did other developers. Many of our customers also submitted feedback to Apple. We pleaded for Apple to restore lost functionality or to provide an alternative solution. We asked in the name of platform potential, productivity, and fair competition, to no avail.
At one point, one of our bug reports was marked with “Potential fix identified.” Yet, a year later, that fix has never materialized.
So we went to work. We re-read the rules of Core Spotlight. Applications have access only to data they themselves submitted. By this rule, Apple Mail has access to mail messages. Now, the Mail application can be extended with plug-ins. To Core Spotlight, such a plug-in is part of the Mail application. The plug-in thus also has access to the messages search index.
HoudahSpot 6.0 now includes a Mail app plug-in. That plug-in is so kind as to share search results with the HoudahSpot application. All you need to do is install and run the plug-in. Your mail messages will again show up in HoudahSpot search results.