[Dovecot] Replication -- multiple users, three or more servers?

[Rich Wales]

I'm running Dovecot 2.2.1 on an Ubuntu 12.04.2 server, with half a dozen
accounts for various family members.  I want to set up replication
involving at least three Ubuntu servers, with different users replicated
on different sets of servers.

For example, I might have mail for "user1" replicated on "server1",
"server2", and "server3"...

while mail for "user2" would be on "server1" and "server2"...

and mail for "user3" would be on "server1" and "server3".

I've read the wiki page (http://wiki2.dovecot.org/Replication), but I'm
still confused.  I'd love to see an example that clearly shows how to
set up specific individual mail users to be replicated on a different
set of servers for each user, like what I described above.

Rich Wales
richw at richw.org