Dear Maintainer, The default value of the destemail configuration in the mail.conf should be only "root" instead of "root@localhost". This is a "standard" used in several debian packages. Mail softwares like postfix when alias root user extenal address ends with: root@localhost.myhost.mydomain.com insted of root@myhost.mydomain.com. Thank you, Dudytz
Sorry for the mistake, the correct configuration file is the jail.conf.
I'm not sure yet if it's a bug, but current fail2ban versions still use this in jail.conf: destemail = root@localhost