search for custom header value in notmuch

Question

Is it possible to search for emails containing a specific custom header value using notmuch?

I know it's possible to search for standard headers like

notmuch search from:alice subject:hello to:bob@example.com

but is there a way to search for something like this?

notmuch search Accept-Language:en-US X-Spam-Flag:NO

Answer 1

It's possible since notmuch 0.29 - see man pages for notmuch-config(1)

   index.header.<prefix> [STORED IN DATABASE]
          Define  the  query  prefix <prefix>, based on a mail header. For
          example index.header.List=List-Id will add a probabilistic  pre‐
          fix  List:  that  searches the List-Id field.  User defined pre‐
          fixes must not start with 'a'...'z'; in particular adding a pre‐
          fix  with same name as a predefined prefix is not supported. See
          notmuch-search-terms(7) for a list of existing prefixes, and  an
          explanation of probabilistic prefixes.

As mentioned in the manual it's stored in the database - similar to custom queries. You have to set it for every header you want to search.

So in the case mentioned in Your question:

notmuch config set index.header.AcceptLanguage Accept-Language
notmuch config set index.header.XSpamFlag X-Spam-Flag

It's possible you would need to reindex your database notmuch reindex '*'

and finally runing the search

notmuch search AcceptLanguage:en-US AND XSpamFlag:NO