Edit the following conf file
/etc/fail2ban/action.d/sendmail.conf
/etc/fail2ban/action.d/sendmail-whois-lines.conf
/etc/fail2ban/action.d/mail-whois-lines.conf
/etc/fail2ban/action.d/mail.conf
Edit the following conf file
/etc/fail2ban/action.d/sendmail.conf
/etc/fail2ban/action.d/sendmail-whois-lines.conf
/etc/fail2ban/action.d/mail-whois-lines.conf
/etc/fail2ban/action.d/mail.conf