It is not unusual for a user to have multiple mailboxes maintained at more than one server. In this case, the proxy maintains a system-wide mapping (common to all users) from non-routable local IP addresses in the range 127.0.0.0/8 to the well-known routable IP address of the remote host running the dovecot server. All the MVA's on the client are then configured to use IP addresses in this range (published by the proxy) to refer their respective hosts.
The proxy binds and listens for connection requests on all the published local interfaces (i.e., all the 127.0.0.0/8 IP addresses configured for the proxy). A request on a given IP address corresponds to a particular remote host (known to the proxy). The proxy can then follow the scheme outlined above to authenticate the user and establish the connection.
