Exim and multiple domains

[Alec Smith <alec@shadowstar.net>]

Using Exim, how can I configure it to process mail for multiple domains? 
Specifically I want mail to domaina.com processed by the .procmailrc in 
User A's home directory while mail to domainb.com is processed by the 
.procmailrc in User B's home directory. Exim is already configured to 
accept mail for the domains, but doesn't do any special processing based on 
the domain name.

At the same time, I'd like to take some mail to user@domainIhost.com and 
deliver it directly to someotheruser on the same machine. Is there a way to 
do this without having to use .procmailrc and process the e-mails twice? 
(ie someuser@domaina.com -> someotheruser and someuser2 -> domain.com are 
mapped in an Exim database -- Something along the lines of Sendmail's 
virtusertable file)

Alec