> The problem at hand is that it is really hard to debug situations like this. Google won't tell you why a particular email is blocked, because that information will be immediately exploited by spammers.

This is a pretty big fallacy, because exactly this approach affects normal users much more than it affects spammers. The solutions themselves would be much different if you have to pass 10 emails per day with 100% certainty, versus a million where a 5% pass rate is good enough.

A better explanation here is that these denials appear to be somewhat random in nature, driven by multiple layers of AI and proprietary systems that may be difficult to explain to the uninitiated, without revealing the whole code of the algorithm into OSS, and it's not quite possible to explain exactly why the AI does the things that it does.

The only solution would be to actually make it possible to troubleshoot this to an actual resolution. Why it's an issue for a domain owner with a dedicated IP address to send a couple of emails per day from their domain name without ending up in Spam is a bigger question here.

> I did a quick scan of rafa.eu.org and found the following:

All the issues you list are non-issues, and merely the perpetuation of the security theatre.

SPF is very useful for a domain like gmail.com, where there are hundreds of individual IP addresses that may be doing the sending, with little ways of finding out if they're legit without SPF. But the domain of the OP has a single server with a single dedicated IPv4 address. It is trivial to verify whether or not the domain owner has authorised a given email sent from such server, and whether it's authentic, even if it doesn't have DMARC, SPF or even MX records, not to mention DKIM, provided that it gets sent from the A or AAAA address of the domain. Yet it's these very emails that get denied, and even putting these useless and unnecessary for the situation DKIM, SPF and DMARC entries doesn't help, either.

Further, your requirement of configuring SPF and DMARC to reject actual forged mail stems from a belief that someone else is abusing the domain reputation to send forged mail and spam, but, (1), that's never been asserted or implied by the sender, and, (2), why isn't Google smart enough to reject those messages then, instead of rejecting these ones that clearly do come from the owner of the domain, due to the match between reverse DNS, forward DNS, MX, EHLO, MAIL FROM et al. Because that's really what's going on here. We're not even talking about mailing lists or forwarding at this stage; the most basic mail setup results in his messages being rejected.

Here and in your another comment in this thread, you're portraying all this advice as expert advice without even having anecdotal evidence to back it up that it actually works; this is sadly a common practice around the internet, to make suggestions for the sake of making suggestions, wasting people's time on implementing solutions that won't make any difference, where the proposed solution has little correlation to the issue at stake. And this is exactly why you get downvoted on the more technical forums like Hacker News for offering this sort of technical consulting for free — because folks here know better, and because these sort of blanket suggestions don't actually do anything for the situation the user experiences, don't solve the problem, and are technically incorrect and superficial to start with.