If you are using Thunderbird 2.0.0.24 then you have the latest V2 upgrade, which was released a few days ago to those who use auto-upgrade.
I've never come across the error you mention. Do you only have one e-mail address and ISP to test with?
If you're using Thunderbird and POP e-mail, drop me an e-mail to keef at keef dot org dot uk and I'll e-mail you back with an e-mail address and settings that you can use on one of my domains, which will be completely separate from your ISP. If that doesn't do the Bayesian thing, you'll know it's your ISP. If it does, it's something in your PC.