In its default configuration, Thunderbird wants to save a copy of each sent message in a folder called 'Sent' which is located in the root of your IMAP store. If the folder does not exist, Thunderbird will attempt to create it. If it cannot be created, Thunderbird will hang until you cancel the send. When you are using a Citadel server, this folder cannot be created because Citadel expects all personal folders to be subordinate to INBOX.
Fear not, however, for the solution is easy. Since the Citadel server automatically saves a copy of each sent message to your 'Sent Items' folder, there is no need to have Thunderbird (or any other client) do this. In other words, you simply disable the client feature altogether since the server is already taking care of that for you. Here's how to do it:
- Tools –> Account Settings
- Select 'Copies and Folders' under your Citadel account.
- Look for the box that says 'When sending messages, automatically…' and observe that there is a checkbox below it labelled 'Place a copy in…'
- Uncheck that box, click OK, and resume your happy day.