Verify your SMTP outgoing email settings
Tools menu and select Account Settings, or click the Application menu button 
and select Options and Account Settings from the sub-menu, and select Outgoing Server (SMTP) from the bottom of the list in the left panel.
Verify the correct SMTP Server is being used
Tools menu and select Account Settings, or click the Application menu button and select Options and Account Settings from the sub-menu, and select the account name. Verify that you are using the right SMTP server for the email address.
Verify that your outgoing email is not being blocked
Verify that your outgoing email is not being blocked by your firewall, antivirus software or your Internet Service Provider
Verify that your mail server supports the minimum version of TLS
Thunderbird 78 expects higher security standards for the so-called TLS network encryption protocol. If your provider does not support the newer versions of the protocol, sending and receiving of messages may fail. You can temporarily lower the bar to check if this is the reason for your problem (which obviously reduces security, so it's not recommended).
