What server settings are you using? In Account SEttings Select the name of the account in left pane Bottom right click on 'Edit SMTP Server' button Post an image showing the smtp server settings.

Select 'Server Settings' for the account in left pane Post an image showing the server settings.

Please check your computer; Firewall Access Firewall which might be the deaful that came with computer or controlled by an Anti-Virus you installed. Thunderbird must be set up as an allowed program.

VPN Please check to see if a VPN has been enabled. There are some Anti-Virus products that provide VPN and it may have been auto enabled. Servers do not like VPN because it hides your IP address, so server think you might be a hacker and therefore block access. Switch VPN off or create a split tunnelling and set Thunderbird to use it.