Thunderbird deleted all my local archives for this account. I then restarted Thunderbird (I couldn't find an easier way to make it re-attempt to connect to the Exchange server). It's possible that Thunderbird had worked after the password change only because existing connections had remained open. I don't know why I still had the old password there. I first need to make Thunderbird forget my saved password: use the “Preferences” → “Preferences” menu, go to the “Security” → “Passwords” tab, click the “Saved Passwords.” button and remove the entry(s) that have the old password. There's no actual way to change the password except at a password prompt (which Thunderbird doesn't show since the server never tells it that the password is invalid). So I need to change the password stored by Thunderbird. If the client presents a valid user name but an invalid password, the server accepts the login, but subsequent commands fail with the aforementioned error message. The message “User is authenticated but not connected” is due to a bug in the Exchange server's IMAP implementation.

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