How to make Exim to send messages with IP assigned to other users is described here. And here in the article you can find some bash scripts for generating files with IP data automatically.
A step-by-step How-To on the subject is inside. With that you'll learn how to configure da_exim and da_dovecot to use Sieve email filters, and how to configure roundcube to manage the rules.
As soon as you enable Pigeonhole with Exim and Dovecot you get email quotas not working any longer. And here is a way how to enable them back with Dovecot.