The DNS tab lets you specify which DNS servers VPOP3 will use. DNS (Domain Name System) is a system which converts Internet host names (eg www.google.com) into a numeric IP address (eg 220.127.116.11), because all connections on the Internet are actually performed using numeric IP addresses, but it is easier for people to remember names.
VPOP3 uses DNS servers for several purposes: accessing spamfilter 'real time blacklists', connecting to remote servers, etc
The Current DNS Servers box, lets you see which DNS servers VPOP3 is currently using. This can be useful if you have told VPOP3 to detect the DNS servers automatically.
You cannot edit this box. If you want to change the current DNS servers, then enter them in the Manual DNS Server List box, and uncheck the Try to detect DNS servers automatically box.
This box lets you specify your own list of DNS servers. To specify multiple DNS servers, separate them with commas.
If this box is checked, then VPOP3 will use the DNS servers which Windows is configured to use. Usually this will work OK, but if the DNS server used by Windows is a bit dodgy (eg on some cheap routers), then VPOP3's extensive use of the DNS service may cause issues, so in that case it may be worth using a different DNS server for VPOP3.
Note that if you have told VPOP3 to access a DNS server outside your LAN directly (eg Google's public DNS servers, or the OpenDNS servers), then you must make sure your router/firewall allows the DNS responses to come back to VPOP3. Most firewalls will do this, but some may not.
This page will periodically do a DNS lookup on a name which exists, and a name which does not exist, and show you the result and response times here.
The results should show 'DNS OK' - if an error is shown instead, then there is a problem with your DNS servers, and you may need to change some settings.
The results ideally should also show a response time of 500ms (half a second) or less. If the response time is more than this, then things like the spamfilter may timeout while waiting for responses for blacklist lookups, giving less accurate results. It will also slow down the processing of incoming mail.