Thx Cthulhu!
For all with the same problem:
Edit /etc/fail2ban/jail.d/postfix-pregreet.local with:
[postfix-pregreet-iredmail]
enabled = true
filter = postfix-pregreet.iredmail
logpath = /var/log/mail.log
maxretry = 1
action = iptables-multiport[name=postfix, port="80,443,25,587,465,110,995,143,993,4190", protocol=tcp]
Save the fail and restart Fail2Ban.
@ZhangHuangbin:
Maybe this is a Debain 10 Buster related problem that should be addressed via the installer?
Greets Mike