Kann nicht gesendet werden, weil Verbindung zu SMTP-Server fehlgeschlagen ist???

You need to know what the server settings are supposed to be and then set up the account accordingly. Which is your email provider? If you post a screenshot of your outgoing (SMTP) server settings, we may verify they're correct. Read the following article in the meantime to see possible causes of the problem: https://support.mozilla.org/kb/cannot-send-messages In particular, if you're using a VPN, disable it and see whether that makes the problem go away. If it does, check whether your VPN has an option called <i>split tunneling</i> which would allow you to let Thunderbird bypass the VPN.