People always ask me how I use Thunderbird to POP mail that I'm "not supposed to"
Answer?
FreePOPs!
Install their exe file, start their service, then set your email client to POP "localhost" (or 127.0.0.1) on port 2000.
If you want to, say, use their program to check your gmail spam folder, just download the special plugin and replace it with the gmail.lau file of the same name in "C:\program files\FreePOPs\LUA\"
Then set your username for that account to (something like) "username@gmail.com?folder=spam"