I wanted to forward information from the director to the backend dovecot (original login name), so I had the userdb on the director return a forward_ologin variable.
However, when I tried to use that variable in the "password_key" query on the backend dovecot, ${forward_ologin} was expanded to UNSUPPORTED_VARIABLE_forward_ologin.
After testing a bit and looking around in the source a bit, and turning on debugging logging, it appeared that the forwarded variables actually appear as "passdb/userdb extra fields". So in my case, the forwarded information is available as %{passdb:forward_ologin}.
So, problem solved, but this is either an implementation bug, or a documentation bug, or oversight.
-- Jan-Pieter Cornet johnpc@xs4all.net Systeembeheer XS4ALL Internet bv www.xs4all.nl