To get to this page, to to Mail Connectors → (choose Mail Connection) → Advanced.
This page lets you set advanced settings for a connection method. The settings on this page are not normally needed.
The Dial-up connections which can also be used for this connection method if already established option lets you tell VPOP3 that if it detects a dial-up connection is in use when VPOP3 wants to use the current connection method, then VPOP3 can just use the detected dial-up connection instead of trying to make a new dial-up connection. This option is only needed if VPOP3 is using a dial-up connection to connect to the Internet and the computer has several dial-up connections configured which can be used for sending/collecting mail.
The Connect through SOCKS proxy server option tells VPOP3 that when this Connection method is used, VPOP3 should make connections through the SOCKS proxy server defined in the Misc Settings rather than connecting directly to the remote servers.
The If this connection fails totally, try another Connection option tells VPOP3 that if all the Mail Senders and Mail Collectors associated with this Connection method fail, then VPOP3 should connect again, immediately using a different Connection method.
One example of where this may be useful is if you normally have VPOP3 connecting through a router, but you also have a 3G dongle in the VPOP3 computer. You could have two Connections defined in VPOP3 - one using the router ('LAN') and the other controlling the 3G dongle using dial-up networking. The normal Connection Schedule in VPOP3 would use the LAN connection, but you could use this option to have VPOP3 connect using the 3G dongle if the LAN connection fails totally.
Note that this option is normally not useful to detect which of two router connections is in use, because VPOP3 has no way of knowing what the router is doing. For instance, if the router has the 3G dongle in it, and the router has fallen back to using 3G instead of xDSL, then VPOP3 has no way of knowing this, so it will still use the LAN connection.