Pop3 can be tested via Command line using the telnet executable (Will need to be selected from Windows features).
Telnet to the Pop3 server in question, this will test Pop3 is accessible, then enter the username and password. As long as the status is OK then it should work. If not the error should provide more information such as invalid username or password.

telnet pop.yourdomain.co.uk 110
user your-emailusername
--Returns Status, should be something like +OK--
pass youremailpassword
--Returns Status, should be something like +OK--