OpenSMTPD and Thunderbird: smtp-in: Bad input on session 5.5.1 Invalid command: Pipelining not supported

While trying to connect to OpenSMTPD running on Debian Stretch Thunderbird couldn’t auto-configure the account and provided the following error in the logs:  
email smtpd[566]: smtp-in: Bad input on session 6567905e3020707c: 500 5.5.1 Invalid command: Pipelining not supported
  This is a Thunderbird config issue, not a server side issue. Change the settings from SSL/TLS to STARTTLS and that should sort it.