[exim] Cram authentication and domain-specific password file

Hello,

I'm trying to configure cram_md5 authentication on my Exim4 mailserver.
The login credentials are stored in a domain-specific textfile with the
format 'username:{CRAM-MD5}<MD5-SUM>'. The path to the password file is
/etc/vmail/users/<domain>.

The problem is, that the credentials need to be looked up in a domain-
specific file. Is this possible at all? When I read the docs for the
cram authentication driver and the field 'server_secret', my impression
was that this might be impossible. Maybe it would be possible by using
server_condition?

Below you find my first attempt, but it doesn't consider a
domain-specific
password filename at all:

vmail_auth_cram:
driver = cram_md5
public_name = CRAM-MD5
server_secret =
${extract{2}{:}{${lookup{$auth1}lsearch{VMAIL_USERS_DOMAINS}{$value}fail}}}
server_set_id = $auth1

Cheers,
jonas