The two most common reasons for this error is: 1. There is a bad/corrupted message on the server. 2. There are too many e-mails on the server. Please login to your Yahoo webmail: http://mail.yahoo.com/ If it is a corrupted message, then it will probably be the very first/oldest message you see in webmail. Read it, then either delete it or move it to another folder. Wait a bit, then tell Thunderbird to check for mail. If that doesn't solve the problem, do the same with each message, from oldest to newest. If you have 100's of messages in your inbox, then I suggest that you delete the ones you already have on your computer (since you are using POP). Also the Trash and Spam folders. First check the spam folder for good e-mails. Then make sure that in your Thunderbird settings, e-mails are not being left on the server. Thunderbird menu: Tools: Account Settings: Select "Server Settings" on the left side under your Yahoo e-mail account. On the right side, in the middle, you will find an option called "Leave messages on server". Un-check it.