[BlueOnyx:21596] Re: 550-5.7.1 this message does not meet IPv6 sending guidelines regarding PTR

[Michael Stauber]

Hi Maurice and Jeff,

> I have the same on 5209R. I have not enabled ipv6 in any way, but my
> provider also provides a ipv6 for the vps. This makes outgoing sendmail
> connections to google's server over ipv6.

That's my experience as well: If CentOS 7 is hooked up to a network that
supports IPv6 (even if that server only has IPv4 configured), then some
services might end up using IPv6 as well. At least for outgoing
connections if the target supports IPv6. In that case it might even
prefer to use IPv6 even if IPv4 is available at the destination.

There are some factors that can throw a wrench into that. Like if you
use a DNS-resolver that only serves IPv4.

There is also the option to disable IPv6 altogether on the OS level:

sysctl -w net.ipv6.conf.all.disable_ipv6=1
sysctl -w net.ipv6.conf.default.disable_ipv6=1

As for reverse records for your IPs: Only the owner of the network block
can set them up. So you might need to prod your ISP for that.

-- 
With best regards

Michael Stauber