What does the error message "[blacklist] could not be tested and will not be used" mean?

This error message means that POPcon tried to test a specific internet antispam blacklist server and found it to tag EVERY ip address as spam source. Since the blacklist queries are actually DNS resolutions this points to problems with the DNS system. This kind of error often happens when either a router is configured to point every failed DNS resolution to some company website or (more often) when the local network card is configured to add a suffix to every query. Please check the tcp/ip protocol settings (advanced) of your local network card for this!

Why DNS? The internet antispam blacklist servers are querried by using DNS lookups. POPcon creates a name to be looked up by combining the IP adress to be checked with the blacklist server name. If the resulting name lookup is successful it means the IP is listed. If the lookup fails (nothing returned) then the IP was not listed.