> On 21 Apr 2016, at 20:46, <list...@tutanota.com> <list...@tutanota.com> wrote: > > What is "unknown" in this case? > > I think it is the RDNS that is not there?
Yes. There’s no reverse DNS for the connecting IP address. > host 65.181.123.80 > Host 80.123.181.65.in-addr.arpa. not found: 3(NXDOMAIN) You should really use dig for DNS troubleshooting. Accept no subsitutes. Well, apart from delv or drill if you’re troubleshooting Secure DNS errors. > I think a good mail server must always have the RDNS, yes? I am not 100% sure > that it is safe to reject this. SMTP connections from an IP address with no reverse DNS is a *very strong* indication of a spam source. But it’s not 100% foolproof, particularly for IPv6. IMO, anytbing that doesn’t have a working reverse DNS entry shouldn’t be sending email.
