I am using the mailcow e-mail stack. Of course I learned the host and all forwarded adrresses. Spamcop has problems parsing the upper received-header which is internal. fd4d:x is an internal and unroutable address (RFC 4193). Interesting is the header underneath, from which the actual sending system can be seen.
Received: from mail.svengo.net ([fd4d:6169:6c63:6f77::b])
by 46850f6954c1 with LMTP
for <x>; Wed, 06 Oct 2021 23:29:47 +0200
Received: from cyan.elm.relay.mailchannels.net (cyan.elm.relay.mailchannels.net [126.96.36.199])
(using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
(No client certificate requested)
by mail.svengo.net (Postcow) with ESMTPS id DC3D01C06D6
for <x>; Wed, 6 Oct 2021 23:29:44 +0200 (CEST)
Parsing headers is interrupted:
host fd4d:6169:6c63:6f77:0:0:0:b (getting name) no name
0: Received: from mail.svengo.net ([fd4d:6169:6c63:6f77::b]) by 46850f6954c1 with LMTP id IPLuNUsVXmFsxAAAxGfoPw (envelope-from <email@example.com>) for <x>; Wed, 06 Oct 2021 23:29:47 +0200
No unique hostname found for source: fd4d:6169:6c63:6f77:0:0:0:b
Possible forgery. Supposed receiving system not associated with any of your mailhosts
Will not trust this Received line.
Mailhost configuration problem, identified internal IP as source
Please correct this situation - register every email address where you receive spam
No source IP address found, cannot proceed.
Re-learning the host does not work. I am currently removing the header as a workaround.