mutt imap_headers

[Nick <debuser@acrasis.net>]

A question about mutt over imap in stretch.  In my .muttrc (what I
think are the relevant lines),

   set imap_headers = "X-SPAM-STATUS"
   set index_format="%H %Z %d %-15.15F %s"
   spam "X-Spam-Status: Yes" "spam"

The good part: when I open a message having the header "X-Spam-Status:
Yes", the "spam" marker appears in the index in place of "%H".

The not so good: no marker appears until I open messages.  I
had expected that my imap_headers setting would cause the marker to
appear on all the qualifying messages in the index.  How can I make
that happen?

Thanks
-- 
Nick