Name: search_index_headers
Type: SWITCH
Default-Value: 1
Last-Modified: 3.0.0

Whether to index headers other than From, To, Cc, Bcc, and Subject.
Experiment shows that some headers such as Received and DKIM-Signature can
contribute up to 2/3rds of the index size but almost nothing to the utility
of searching.  Note that if header indexing is disabled, headers can still
be searched, the searches will just be slower.
