I am getting the following error when I try to send an email: Sending of the message failed. The certificate is not trusted because it is self-signed. The configuration related to <smtp server name> must be corrected. The server uses a self-signed certificate. Here are the important settings: Port: 587 Connection Security: STARTTLS Authentication Method: Kerberos / GSSAPI Thunderbird is able to retrieve emails from the same server (IMAP / 143 / STARTTLS / Normal Password) and it showed me the Confirm Security Exception dialog first time it connected. But it is not showing me the dialog when sending emails. Needless to say that manually adding exception is not working with the error: No information available Unable to obtain identification status for this site So how do I fix this? Note that I am able to send email from an older version of Thunderbird using exact same server and settings. Thunderbird Version: 78.3.1 Build ID: 20200925001233