This error happens when you have multiple TCP/IP subnets on your network. The computers which are on different subnets from the VPOP3 computer will not be able to see VPOP3 whilst VPOP3 is connected to the Internet (through a dial-up connection). Whenever VPOP3 is offline, it will work correctly.
This error is actually a TCP/IP routing problem rather than a VPOP3 problem. To confirm this, try using the PING command to contact the VPOP3 computer from a computer experiencing this problem. The PING will work when the VPOP3 computer is not connected to the Internet, but not work when it is.
The fix for this is to correctly set up the TCP/IP routing table on the VPOP3 computer. Simply setting the TCP/IP Default Gateway setting is not sufficient, because when the computer connects to the Internet, the ISP will set another Default Gateway which will temporarily override the one set in the control panel.
The easiest way to explain how to set the routing table is by use of an example. Assume that the VPOP3 computer is on the TCP/IP subnet 192.168.1.xxx and the other subnet is 192.168.2.xxx. The router connecting these two subnets has addresses 192.168.1.254 and 192.168.2.254.
On the VPOP3 computer you should go to a DOS prompt and run:
ROUTE ADD 192.168.2.0 MASK 255.255.255.0 192.168.1.254
If VPOP3 is running on a Windows 95/98 computer, you should put this in a batch file and run it from the Startup group. If VPOP3 is running on Windows NT/2000, you should add a -P' after the ROUTE and before the ADD, to make the setting permanent.