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. 

Server status

The ps command can be used to determine if your system is using init or systemd. If PID 1 is init, then you will use the service command. If PID 1 is systemd, then you will use the systemctl command.

If your system is using systemd, use the systemctl command to ensure postfix and dovecot are running.

systemctl status dovecot
systemctl status postfix

If your system is using init, use the chkconfig and service commands to ensure postfix and dovecot are running.

service dovecot status
service postfix status

Firewall

On both the email server and client, ensure the appropriate ports (25, 143, 465, 587, 993, 995) are open in iptables or firewalld.

ping

On the client, use ping to ensure you can get a response from the mail server.

ping mail.example.com

Username

On the email server, ensure john.doe exists in the /etc/passwd file.

Password

On the email server, use the passwd command to ensure the password is correct.

Interfaces

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

SMTPs

If the email server is configured to use SMTPS, ensure you can make a SMTPs connection to the Postfix server.

POP3s, IMAPs

If the email server is configured to use POP3S or IMAPS, ensure you can make a POP3s or IMAPs connection to the Dovecot server.