[Exim] Multiple logins?

My ISP grants me multiple mail boxes, and each has its
own login identity. I mirrored the account/usernames
on my server for simplicity's sake and I managed to
figure out how to login to and authenticate one
account, now I need to get Exim to handle each account
in its own way. How to I get the user's name or the
sender's name from the email? I'd like to extract
that and use it in an lsearch to get the password.

Robert