I've noticed that a lot of my spam is ending at a 100.x.x.x address, for example https://www.spamcop.net/sc?id=z6743104032z068b251bb86596b30e7fe37963fd992fz:
The 100.64.0.0/10 address space is "Shared Address Space", intended for ISP internal use, such as in career-grade NAT.
I thought that this was spammer spoofing of the headers, but when I look closer, I see that this is Apple internal iCloud routing. I ran the mail hosts test, and the probes from SpamCop to me have in part:
Received: from mr85p00im-ztdg06021201.me.com (mr85p00im-ztdg06021201.me.com [17.58.23.189])
by mr85p00im-ztdg06021201.me.com (Postfix) with ESMTPS id CFA8E321282
for <xxx@xxx>; Tue, 1 Mar 2022 20:51:55 +0000 (UTC)
Received: from unknown (unknown [100.108.117.178])
by mr85p00im-ztdg06021201.me.com (Postfix) with SMTP id AAA47320EB5
for <xxx@xxx>>; Tue, 1 Mar 2022 20:51:55 +0000 (UTC)
Received: from mr11p00im-smtpin012.me.com by p28-mailgateway-smtp-5c9bd88869-2jm44 (mailgateway 2209B259)
with SMTP id ac8a1f75-aeb2-42f2-ade8-1afa28cf6452
for <xxx@xxx>>; Tue, 1 Mar 2022 20:51:55 GMT
Received: from prod-sc-www03.spamcop.net (vmx.spamcop.net [184.94.240.112])
by mr11p00im-smtpin012.me.com (Postfix) with SMTP id EC87727F9540
for <xxx@xxx>>; Tue, 1 Mar 2022 20:51:52 +0000 (UTC)
So, we can see that the 100.108.117.178 header is within iCloud, and should be skipped over on the way to finding the real spam source.
I forwarded this through to the mail host configuration, but it isn't showing any 100.x.x.x addresses in the mail host list.