Maybe if you share account info and settings for the SMTP server setup, someone here can assist. Please post all the info from that pane.

Description: madasafish

                     Server name: mail.totalise.co.uk
                                  Port : 587
                       User name : mikewilson@totalise.co.uk

Auththentication method : Password, trsmitted insecurely

       Connection security : none

My wife's is the same except username which is ingridg@totalise.co.uk I must emphasise that we have been using Thunderbird for years; we are not techies and have changed nothing since initial installation and that the problem happened for the first time on Friday when an update to Thunderbird was installed (automatically)

Email providers have been implementing tighter security this year. I suggest you establish connection security for the account, such as STARTTLS. If that doesn't work, try SSL/TLS. Report back on any problems.

Sorry, that doesn't mean anything to me

click 'account settings' and then scroll down to 'outgoing servers SMTP' and list your server and click the edit button to fix it. If not sure of next steps, please post a screenshot of that pane. Thank you.

The problem has now gone away. Thank you for your help