If a LAN forwarding destination server fails, you may wish to change the LAN forwarding rules to point to the new server. (Note that, if planning ahead, it may be easier to use DNS host names as the targets, so you can simply change the IP address associated with the server host name instead)
As this database query updates the database, be sure to take care doing it as you can damage your VPOP3 installation if you do it incorrectly. Make sure you have a database backup before performing the query.
If you want an extra bit of safety, see the running database updates within a transaction article.
At a command prompt in the VPOP3 directory run
(The password is 'vpop3pass')
UPDATE rules.lanforwarding SET server='<new server>' WHERE server='<old server>';
Where <new server> and <old server> are the IP address (and optional port number if not 25) of the new & old LAN forward target servers respectively. Don't include the <>.
UPDATE rules.lanforwarding SET server='192.168.1.53' WHERE server='192.168.1.52:587';
If you have SMTP authentication on the new server, you can use
UPDATE rules.lanforwarding SET server='<new server>', username='<username>' password='<password>' WHERE server='<old server>';
N.B. VPOP3 does not need to be stopped while you do this