[exim] Authenticator Problem

Hello,

I'm new to exim and I have a little problem with configuring an
authenticator for my mail server. The aim is to auth against a cdb file.
The key of the file is the full mail address (like adam@???).
The value is a string containing multiple values divided by ":". The
first argument of this string is the password for authentication. The
server should lookup the users name and check if the given password
equals the one in the cdb file. My authenticator looks like this:

auth_plain:
     driver = plaintext
     public_name = PLAIN
     server_prompts = :
     server_condition = # don't know what to do here
     server_set_id = $auth2

I have searched the net to get a solution but didn't find anything. I
hope you can help me. Thank you.

Best Regards

Stephan Schmidt