Change the from address (email sender) in fail2ban:
All of my fail2ban mails were sent from fail2ban@ „loftXXXX.startdedicated.de“ instead of „mattionline.de“
In my jail.local i have configured
sender = fail2ban@DOMAINNAME.TLD
but even with this line the mails are coming from the startdedicated address.
In /etc/mailname was configured startdedicated. After i changed this file to „mattionline.de“ it worked.