Here's the deal - I always have "leave messages on server" checked on my POP3 email account so I can check mail from multiple computers. I'm using Thunderbird, and I checked the "leave until I delete them from inbox" box as well. Well, it doesn't exactly delete them from the server when I do...
FYI you can make that stupid extension cable usable by drilling out the plastic tab with a small drill bit and crimping the metal down into the drilled out hole with some needlenose pliers.