Incoming messages are received through spamfilter01.heinlein-hosting.de:
Reçu: from smtp1.mailbox.org ([18.104.22.168])
by spamfilter01.heinlein-hosting.de (spamfilter01.heinlein-hosting.de [22.214.171.124]) (amavisd-new, port 10030)
with ESMTP id HQk53i3QR5up for firstname.lastname@example.org
As a result, DMARC check fails on incoming messages:
Authentication-Results: default_out; dmarc=fail reason="" header.from=yyyy.com (policy=reject); spf=fail (default_out: domain of email@example.com does not designate 126.96.36.199 as permitted sender) firstname.lastname@example.org
Shall I add spamfilter01.heinlein-hosting.de to the SPF list of permitted senders? The problem is that you don't seem to be managing the server spamfilter01.heinlein-hosting.de.
So what do you recommend?