[Exim] authentication string question

I am going to have a very messy authentication section, I can feel it.
Here's my initial question - I am going to have users auth'ing as
user@domain. I need to cut that string up into user and domain - user
will be the key I'll be looking up, and domain will specify which passwd
file I need to use. As far as I can tell, all of this needs to be done in
the server_condition line. Is it possible to split this up this way? I'm
not very familiar w/ exim's string functions because I usually can rely on
static strings.

Thanks for any hints
--John