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Thank you in advance for your patience and understanding on the matter.
Looks like your reverse DNS doesn't match the SMTP sender, which usually is the host name of your server. The IP for mail.sitecolor.de is 146.0.42.97, which in turn resolves to the reverse DNS of yemindo.de. That's a mismatch.
Looks like your reverse DNS doesn't match the SMTP sender, which usually is the host name of your server. The IP for mail.sitecolor.de is 146.0.42.97, which in turn resolves to the reverse DNS of yemindo.de. Thats a miss match.
This is from the result from a mxtoolbox test right? Are you also having trouble with sending email or are you only concerned with the warning from the mxtoolbox test? From what I can gather this warning can often be ignored as it does necessarily indicate a problem.
If you don't have any trouble with sending email you could leave it as is. Otherwise you'll have to change the host name of your server (and update the rDNS accordingly).
I have run into the same problem and checked the RFCs, but found nothing that would require a 3rd level. See also Definition of Fully qualified domain name. So I'm pretty sure this is mxtoolbox overreacting.
This question above has a specific answer for this specific situation above.
But, I have another very similar situation. The same issue is here, but the claim is that "mail.domain-name.com" reverse DNS is not a valid Hostname, but the mail. DNS and the mail. part of the address is never used.