wl-fcc not working with smtpmail-send-it

Hello list,

I have configured Wanderlust to send email using smtpmail-send-it, as that was
the only way to make it cooperate with SUBMISSION at port 587. However I can't
make FCC working, in particular I want to file the sent messages directly into
an IMAP folder. Here are the relevant parts from my .wl config:

(setq wl-draft-send-mail-function 'smtpmail-send-it)
(setq
      wl-fcc                    "%gmx.sent"
      wl-fcc-force-as-read      t
      )


While writing this message I see the header is properly added at the headers:


Fcc: %gmx.sent


However nothing appears at that folder. IMAP debug logs show no operation at
all. Any ideas how to make it work?

Thanks in advance,
Dimitris