[exim] expand server_secret

Hi!

I want to set up a cram_md5 authenticator.
The problem is that the users are identified by the user/pass couple, not
only by user@domain. So it is possible to have 2 or more users with the same
username but different passwords.

I tried this:

server_secret = ${lookup mysql{SELECT U_Password FROM Users WHERE
U_Mailbox='${quote_mysql:$1}'}{$value}fail}

but this fails for usernames which are used more than one time for all users
but one.

So I went over to this:

server_secret = ${lookup mysql{SELECT U_Password FROM Users WHERE
U_Mailbox='${quote_mysql:$1}' AND
U_Password='${expand:server_secret}'}{$value}fail}

as the documantation says: "Type: string, expanded" I hoped, I can get the
server_secret on the right side of the '='. But no chance, nothing is
expanded :(

Is there any possibility to get the server_secret in the mysql-statement?

TIA,
Max