In an earlier thread, Alexzorg reported that his POP3 server had been blocked by what he finally traced to the avast! Mail Shield. He says he has experienced this problem for several days.
I would like to report that starting yesterday afternoon I began experiencing the same problem. I could not connect with my POP3 server using Thunderbird, Windows Mail or telnet. I was able to ping the server, but not connect to port 110. After several hours of troubleshooting between myself and my mail provider's support staff, I finally pinpointed the source of the problem as the avast! Mail Shield.
Completely turning off avast!, turning off all shields, and turning off the Mail Shield alone all make the POP3 server accessible. Please note igor, whether you believe it or not, the Mail Shield is blocking access to servers.