Originally Posted by MET
Both of the two mail-addresses are valid ones.
Sender may not work anymore, depending on your system and installed mail or mailx.
Check your jail.conf for
# email action. Since 0.8.1 upstream fail2ban uses sendmail
# MTA for the mailing. Change mta configuration parameter to mail
# if you want to revert to conventional 'mail'.
#mta = sendmail
mta = mail
and maybe try mta = mail