gnus-message-archive-group does not work without Gnus

[Teemu Likonen]

I have mail-user-agent set to gnus-user-agent:

    (setq mail-user-agent 'gnus-user-agent)

This works. "C-x m" (compose-mail) opens message-mode buffer. I also use
nnmaildir back-end to store sent mails, so I have configured it like
this:

    (setq gnus-message-archive-group "nnmaildir:sent")

It works but only if I have started Gnus before. Sadly it doesn't work
when I start clean Emacs and do "C-x m" (compose-mail) right away. There
is the Gcc header in the buffer but after sending the mail the echo area
shows this error message:

    Can't open server nnmaildir

The mail is sent correctly but it's not archived anywhere.

So, is it possible to get my nnmaildir:sent Gcc work with message-mode
even if I haven't started Gnus yet with "M-x gnus"? How?