[AusNOG] IPv6 reverse DNS and Mail ...

[Jeremy Visser]

On 20/05/2013, at 1:55 PM, Terry Sweetser (SkyMesh CTO) <terry+AusNOG at skymesh.net.au> wrote:
> At SkyMesh, we routinely get "submissions" from remote mail admins to stop blocking their email.
> 
> It turns out on many occasion that the remote site has ipv6, no reverse dns, and is connecting to our MX farm on ipv6.

I see this most often on 6to4 which some routers either set up by default or make it very easy to turn on (Apple AirPort, I'm looking at you — yes, some people run these in front of a mail server).

I suspect this is the reason why MS Exchange defaults to IPv4 outbound e-mail, even if IPv6 is available. The only way I've figured out how to convince Exchange to send out via IPv6 is by sending to an IPv6–only MX.