On Wed, 19 Oct 2011 19:14:16 -0400, Richard Stallman <rms@gnu.org> wrote:
> Is it feasible to use notmuchmail to search old mail,
> but use Rmail to read incoming mail?

Hi, Richard.  Notmuch does not modify any messages, so it can be used in
parallel with other mail agents without problem.  Indexing your mail
with notmuch should not affect interfacing with your mail store with any
other MUA.

I'm also quite sure that the notmuch emacs interface could be used in
parallel with other emacs-based mail clients as well.  The notmuch emacs
interface manages it's own set of buffers, and I don't see any reason
why you couldn't be doing your normal Rmail in one buffer, and
conducting notmuch searches in another.

hth.  If you have any other specific questions, let us know.

jamie.