Suddenly without warning Thunderbird displays a message 'Checking mail server capabilities.......' in the bottom left of the inbox screen. Messages from the server [Optus] are not downloaded. I have changed the SMTP server to the default 465 and changed the security setting to 'None' but the problem has not been resolved. I note that there are alot of queries about this problem but I cann ot find a solution anywhere on the Mozilla help pages.