[Exim] Using ldap as a sender_address_relay search method

I'm trying to figure out a way to get an ldap search for
sender_address_relay but I can't find anything in spec.txt about what
variables are available at that time to use as a filter for the search.
The way I would like it to work would be by matching mail=xxx@yyyy
against the mail from:

Is this possible?

-- 
  Artificial Intelligence stands no chance against Natural Stupidity.
            GAT d- -p+(--) c++++ l++ u++ t- m--- W--- !v
                 b+++ e* s-/+ n-(?) h++ f+g+ w+++ y*