Hello Matthias,
tanks again for mentioning this.
By looking at /var/log/rspamd/rspamd.log, 5 messages of you were rejected within 2020-03-22 10:09:34 GMT to 10:20:43 GMT as they probably contained a link to humdi[.]net, which resolves to 2a01:7e00::f03c:91ff:fe37:3421. This IPv6 address is currently listed at SBL CSS due to sending low-reputation messages. Details can be obtained from: https://www.spamhaus.org/css/removal/record/2a01:7e00::f03c:91ff:fe37:3421
Since I neither want to request a delisting of that IPv6 address nor to disable RBL lookups for resolved IP addresses as they are pretty effective and we would otherwise have trouble to deliver your email to all mailinglist recipients, may I ask you to simply avoid the domain humdi[.]net in your messages?
By the way: For testing purposes, a mailinglist is available at test@lists.ipfire.org. :-)
Thanks, and best regards, Peter Müller