Mailboxes synchronization architectures
With the IMAP synchronization feature, your a able to create en central mail server that is able to centralize remote mailbox into the same account.
You can decide to create a synchronization system that able to update remote mailboxes from the local mailboxes changes.
In this case, users can read the same content from any mailboxes.
You can provide a kind of IMAP synchronization gateway that is designed to be stored on a DMZ and replicate mailboxes between 2 servers.