The messages "SELECT Command is not valid in this state" is symptomatic of this being a yahoo account. That's a surprise, as bt.com have been slowly changing to an alternative provider. The combination of bt and yahoo is rather scary. Neither is best of class in email; put them together and you get a real mess. Even with an account switched over to the other provider (critical path), my btinternet.com accounts are rather unreliable. Is it an IMAP-based account? If so, I'd suggest you remove it from Thunderbird, and set it up again. If it's possible, the new instance of your account should go through the new provider. If it's POP, don't remove it until you have taken measures to ensure its messages will persist. Move them to another account or to the Local Folders account in Thunderbird before removing this account. Actually, regardless of the account type, I'd copy all the messages to Local Folders before doing anything that might break the existing account.