I added 127.0.0.1 to /etc/fail2ban/jail.local even if there already was a DEFAULT entry with ignoreip for localhost:
[postfix-iredmail]
enabled = true
filter = postfix.iredmail
action = iptables-multiport[name=postfix, port="http,https,smtp,submission,pop3,pop3s,imap,imaps,sieve", protocol=tcp]
logpath = /var/log/mail.log
ignoreip = 127.0.0.1 127.0.0.0/8 It works again.