The [postfix-sasl] jail do not work as expected. We need to modify the jail file: /etc/fail2ban/jail.local and replace the [postfix-sasl] section with:
[postfix-sasl]
enabled = true
backend = systemd
maxentry = 3
bantime = 1h
filter = postfix[mode=auth]
port = smtp,465,submission,imap,imaps,pop3,pop3s
#
Restart fail2ban with systemctl restart fail2ban, now SASL logins should be blocked.
