These instructions should work for any recent Thunderbird (version 68 and up)

• Tools → Account Settings → Add Mail Account
• Your name: Your Name (ex. Bill Gates)
• Email address: your email address (ex. amrowsell@sdf.org)
• Password: your normal shell login password
• Click Continue and then click Manual Config
• Incoming IMAP
• Incoming Server Hostname: mx.sdf.org
• Security: STARTTLS
• Port: 143
• Authentication: Normal password
• Outgoing SMTP (only works if you are MetaARPA or pay for dialup, VHOST, or VPM)
• Outgoing Server Hostname: mx.sdf.org
• Port 587
• Security: STARTTLS
• Authentication: Encrypted password
• Username: Your domain name, @sdf.org … for example, amrowsell.sdf.org@sdf.org
• Click Re-Test and Create Account
• You will be prompted for your SMTP password the first time you go to send mail. This is the password generated by mkvpm on the command line.