Thunderbird has been reporting that I'm using 81% of my inbox quota, but my ISP says I have no inbox quota. One of them is wrong. How do I go about figuring out who?
It is unlikely that your ISP really has given you an unlimited inbox. Perhaps they've assigned a certain size and will increase it as needed if you use up what they give. If your inbox starts taking up several gigabytes or so, I'd expect for your ISP to talk to you about your usage.