Trouble with extra headers

[N. Jackson]

I'm trying to use gnus-extra-headers and I don't seem to quite have the
trick of it.

I have a header "Delivered-To" on my local IMAP server which identifies
which mail server a message was retrieved from. I want eventually to use
the value of this header in Gnus select methods to chose my "From"
header and outbound smtp server for replies.

As an initial test, I wanted to try limiting my Group Summary buffer
using the value of this header, and I wanted to try displaying the value
of it in my summary line.

I added

    (setq gnus-extra-headers '(To Newsgroups Delivered-To))

to my .gnus.el file. (Eventually I would have this in the group
parameters for my local IMAP server, but it seemed simplest to add it
globally at first.)

Then, in my Summary buffer I try to use the /x command to limit by an
extra header. This prompts me for the header to limit by:

    Limit extra header (default To):

and I can choose "Delivered-To" using tab completion, so clearly the
change to gnus-extra-headers "took" at some level. (Previously
"Delivered-To" wasn't accepted here.) But then at the subsequent prompt

   Limit to header Delivered-To (regexp):

whatever I enter seems to result in no matches. For example if I simply
enter the leter "a", the response is "Found no matches for
"a"". Although I would think this should match for example at least all
my accounts on gmail as "gmail" has an "a" in it.

I checked the headers of a few emails in the group by opening them and
using C-u g to show all headers, and the "Delivered-To" header is there
and contains nothing unexpected. For example, here is one:

    Delivered-To: nljlistbox2@gmail.com

I thought I might have a regular expression problem, but when I do the
same thing with the "To" header, I get my Summary buffer "limited" as
expected.

What could I be missing?

I also tried to add the value of the Delivered-To header to my summary
line. I changed

    (setq gnus-summary-line-format "%U%R%z%I%(%[%4L:%&user-date;
    %-23,23f%]%) %s\n")

to

    (setq gnus-summary-line-format "%U%R%z%I%(%[%4L:%&user-date;%~(form
    (gnus-extra-header 'Delivered-To))@ %-23,23f%]%) %s\n")

but it makes no difference (or possibly adds a space or two).

Does this suggest that Gnus is not reading/saving the values of the
"Delivered-To" header when it retrieves the headers for my mail, and
only retrieves the value when I open the mail?