[exim] suggestion on exim configuration

Hi list,

the firm where I work wants to migrate from an obsolete Exchange server
to something newer and more performant. I thought that exim could be a
good solution and they want to integrate it with spamassassin, an
antivirus software like clamav and webmail.

At the moment I am concentrating on the exim configuration, it has to
handle multiple domains. A simplified scenario should be like this,
three users (user1, user2 and user3) and two domains (domain1, domain2)
so that

user1@domain1
user1@domain2

are two different mail accounts (don't receive the same mail) and

user2@domain1
user3@domain2

receive the same mail.

Until now I have experience with basic exim configurations so I need an
advice on how to behave the right way for a production mail server
handling multiple domains and users. How should I create accounts to
handle this situation? Is it better to create machine accounts or
authenticating against a DB server? Thank you for possible answers.

Fabrizio