Re: [tlug] [slightly OT] SMTP and the internet, protocols and the internet

[Christian Horn <chorn@example.com>]

On Mon, Jan 11, 2021 at 02:43:08PM +0100, Christian Horn wrote:
> 
> On Mon, Jan 11, 2021 at 12:24:07PM +0000, Schwartz, Fernando G. | @SCA wrote:
> > 
> > Further, if you check at < https://mxtoolbox.com/diagnostic.aspx >,
> > 
> > 220 fluxcoil.net ESMTP Postfix (Debian/GNU) is missing a " rDNS " to the IP.
> > That's probably the main issue.

Hm.. the tool says "Reverse DNS is not a Valid Hostname", but no
details.

I thought I had seen an issue, but it looks ok to me.
I had seen 'dig -x 45.136.30.123' returning 'fc' instead of
'fluxcoil.net', turns out that is a "feature" of 
systemd-resolved.

Since Fedora33 it is default.  I have
  nameserver 127.0.0.53
in /etc/resolv.conf, and requests like done with 'dig' are 
running through systemd-resolved.  The "feature" is that it
used an entry like
  45.136.30.123 fc
in /etc/hosts to return then "fc".. interesting.
But I'm not sure what they think how rDNS should be broken.

t-online was also seein rDNS in order, that was not the
issue leading to rejecting the mails.

Chris