You may face such error when you try to connect to the email server ( postfix ) from your email client, especially when you use the alternate SMTP port. To fix this error, open /etc/postfix/master.cf using a text editor, scroll down / search for a line that starts with “submission inet n – n”. In the particualr line, change “smtpd_tls_security_level=encrypt” to “smtpd_tls_security_level=may” . Now save the file and restart postfix service. That will fix this issue.