It’s likely misconfigured skim/dmarc/spf. Perhaps @Alexey can send test email from a forum to some cheker tool like https://www.mail-tester.com/, and get verbose diagnostic.
You can copy source of that email as received by the mail-tester from that test page and paste it (in its entirety, otherwise dkim signature will break) to analyzers, like www.appmaildev.com or mxtoolbox.com to see more details.
The problem is that DKIM signature domain is not aligned, (it’s actually yellow on mail-tester; but mailtester is quite relaxed, 10/10 does not mean much, it sometimes ignores tests that it can’t check. you need to read actual results)
d domain is forum.storj.io, but the envelope from is literatehosting. I.e. DKIM passes, but for the wrong domain, and DMARC will fail, most properly configured mail recipients will drop it, as this is precisely what spammers do.
No, that would only matter if the message would be getting to junk folder due to high spam score. Here it seems the mailserver rejects it outright, even before routing it to recipients box. I don’t know where did @Bivvo get that logs from, there there could be more information there, or perhaps storj shall open the case with Apple, but I’m sure they’ll just point out the misaligned DKIM and close the case as “behaves correctly”
I’m also using iCloud mail, albeit with my custom domain. i.e. arrogantrabbit.com’s MX is iCloud, and I have not once received an email from storj forum, since 2022, but at that time emails were not routed via literal hosting, and from and dkim domain were aligned (forum.storj.io)
If this could help, my E-Mail server often rejects Storj-Mails too. (Only the Forum mails, but not Node Online/Offline Mails).
Here is the Rspamd Result:
CS01 from iCloud stands for “Content Scanner”, that means there is something in your message that’s hitting their internal filters, it can be wording it can also be an outgoing link. It has nothing to do with authentication or configuration of your mail server. You can email Apple postmaster and sometimes they will give you clues as to what triggering it or even straight out tell you. They are actually pretty nice, compare to most mailbox providers. Sometime they will adjust their filters and it won’t trigger again.
This is the only eMail Tester that really analysis the content. But it’s worth contacting Apple’s postmaster they usually respond within 24 hours. I never ever got a negative response from them, don’t be afraid to email them.