Why is that? In those two particular cases I don’t think the problem was ignorance concerning RFCs, but a wrong or yet not completely done configuration:

As an example: the host mail.example.com has an A-Record to the IP adrress 1.2.3.4 . And 1.2.3.4 has a correct reverse PTR entry for mail.example.com. BUT:
1.2.3.4 (well, because we are talking about reverse lookups here that is of course 4.3.2.1.in-addr.arpa) also has another PTR entry to mail.example2.com.
BUT: mail.example2.com does NOT have an A-Record that resolves to 1.2.3.4. In my cases it just didn’t have any A-Record.

So what happens is that the mailserver (in my case postfix) will do a reverse lookup of the IP. As a result it will get mail.example.com OR mail.example2.com randomly. If you specified reject_unknown_client_hostname it will try to match that result it got with the IP address the connect comes from doing a forward lookup.
Meaning: If the result of the first lookup was mail.example.com it will find a correct A-Record that resolves to the IP. All is fine.
But if the result of the first lookup was mail.example2.com it won’t find a matching a record and reject the email.

I find that really annoying. I mean, errors happen, but until now, only one of them has replied to my email containing information on the problem.

To everyone out there that has a mailserver: PLEASE configure your DNS entries properly ;)

For those who want to see what I mean live:

Try comspot.de or dataworld.de ;)

Dataworld.de has told me they will check the issue with their Administrators :) So if you find the error being corrected please tell me about it.