This error appears when attempting to set up a new email account in Thunderbird.
Let's use an example where Postfix is the SMTP server and Dovecot is the POP3 / IMAP server.
On the Postfix/Dovecot server, ensure both Postfix and Dovecot are active and running.
~]# systemctl status postfix ~]# systemctl status dovecot
On the client, use ping to ensure you can get a response from the mail server.
On the email server, ensure john.doe exists in the /etc/passwd file.
On the email server, use the passwd command to ensure the password is correct.
On the email server, in the /etc/postfix/main.cf file, set inet_interfaces to all, to allow remote hosts to be able to connect to the Postfix server.
inet_interfaces = all
If the email server is configured to use SMTPS, ensure you can make a SMTPs connection to the Postfix server.
If the email server is configured to use POP3S or IMAPS, ensure you can make a POP3s or IMAPs connection to the Dovecot server.