This means that the ISP POP3 or SMTP server which you have told VPOP3 to connect to does not currently exist. This might be that you have mistyped the server address, or that your ISP is having temporary problems, or that you are using an internal DNS server or proxy which is either slow or not configured correctly.

The first thing to do is to go to the VPOP3 settings, then External Mail → In Mail (if the error is a 'POP3 Client Error') or Out Mail (if the error is an 'SMTP Client Error'), and check the server addresses are correct - contact your ISP if you are not sure.

Also, try PINGing the server addresses from a DOS prompt on the VPOP3 computer - this will show if Windows on that PC is able to resolve the server names to numeric IP addresses correctly.

If VPOP3 is connecting through a router or proxy server, then the DNS server settings on the VPOP3 computer (in Control Panel → Network → TCP/IP) could be set incorrectly as well.