It is quite common for the SMTP and POP3 servers to be the same box so mail is delivered and collected from a spool directory controlled by a single localised system. In a situation where the SMTP daemon is incapacitated it is desirable to switch to the backup server so users can still send mail. Even if the POP3 daemon was still operable by switching to a backup server, that invariably does not have access to the mail spool and so POP3 also becomes unavailable. However a POP3 Switch can overcome this.
A POP3 Switch is simply a data pipe that accepts a list of foreign host/port pairs and tries them in turn until a connection can be made as shown in Figure 4. So in our situation the POP3 Switch may first try to contact the POP3 port of the lame host and then go to a dummy POP3 server listening on a port on the local host.