This may be caused by an e-mail on the server being corrupted. Log into your webmail using your web browser. Empty your Junk/Spam folder (checking it first for good e-mails of course). Try getting messages in Thunderbird again. If there is still a problem, then in webmail, find the oldest message in your Inbox and either delete it or move it to another folder. Then try getting messages again in Thunderbird. Keep doing this until you stop getting the error in Thunderbird.