[Exim] User /and/ domain?

I was not able to find this issue addressed in the archives so decided
I'd better ask here.

If it possible to have exim validate the user as well as domain on the
MAIL FROM: line? I see the 'sender-verify' option, but it does not seem
to do what I need.

Specifically, the system in question uses a database which contains a
large list of users and multiple domains for which various users are
valid. I would like to have exim reference this database (possibly
calling an external command that returns 'true' or 'false' if need be)
before deciding that a sender is 'valid' and accepting the message.

Thanks for any info

Jason