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?

Hello,

notmuch can only index and search through mail that is stored in a
maildir[1], but not in an mbox file. As far as I can tell from the
manual, rmail can fetch mail from a maildir directory, but it will
delete mails from it after doing so.

If rmail can be configured to leave the files in the maildir after
copying them (without ending up with duplicate mails in rmail the next
time it fetches mails), then notmuch and rmail should be usable in
parallel. (Although information such as unread flags etc. will 
not be synchronized back from rmail to notmuch).

Another option would be to convert mbox files containing old mails to a
maildir using something like mb2md[2] and have notmuch index only those
directories. That way, old mail will be searchable with notmuch, but only
after explicitly archiving them this way.

[1] http://cr.yp.to/proto/maildir.html
[2] http://batleth.sapienti-sat.org/projects/mb2md/

Cheers,
Daniel