thanks for your response. I am evaluating notmuch / xapian for building an application for analyzing in various ways a fairly large number of emails accumulated over several years. I am afraid the number of headers that would ultimately need to be indexed is therefore quite a lot larger than what notmuch currently indexes.

2012/10/1 Austin Clements <amdragon@mit.edu>

Quoth Petri Savolainen on Oct 01 at 3:39 pm:

> Hello,
> I could not find information anywhere in notmuch docs about what is
> actually indexed - specifically, what email headers are indexed and
> searchable? If a header is not indexed, does searching for its value still
> result in a search hit?
> It would be nice if one could just provide the list of headers to be
> indexed in some configuration file or something.
> Thanks,
> Petri

notmuch doesn't currently implement this, though it is an
oft-requested feature. One (not insurmountable) difficulty is that
the database would have to be rebuilt if a user-configured list of
headers changed and there are technical limitations that prevent us
from simply indexing all headers. Out of curiosity, what headers are
you interested in indexing?

The currently indexed headers are described in man
notmuch-search-terms.