Restricting my reply to one tiny bit of your mail:

You wrote:
> non-recursive is the only thing that makes sense for Maildir++ folders

Either I'm not understanding Maildir++ folders, or I don't agree with
you.

I might have an email archive that looks like this:

  Maildir
    .work
      .project1
      .project2
      .etc...
    .family
      .dad
      .mom
      .brother
      .etc...

With the above setup, what would be unreasonable about wanting to search
for all work-related messages (across all projects, say) with a string
like "folder:work" ?

Now, a person might definitely want to search for messages in the
".work" folder directly, (not including the sub-folders), so we should
provide support for users to get at that behavior as well, (such as a
proposed "folder:work$" or so).

To me, both cases are perfectly legitimate, and I don't understand an
argument that claims that only one makes sense. (Or again, I may be
misunderstanding something.)

-Carl

-- 
carl.d.worth@intel.com