MX servers refusing mail from host via ipv6

Doug McIntyre merlyn at
Fri Oct 1 20:41:55 UTC 2021

On Fri, Oct 01, 2021 at 02:37:08PM +0200, Paul van der Zwan wrote:
> It looks like the servers are refusing mails from unresolvable hosts:

As much as I think it is worthless security, this has been the
standard for quite some time on IPv4, and IPv6 copied it along. I'm
not sure you'd find more than a handful of mail servers out there that
would let a mailserver without a reverse PTR setup to talk to them
either on IPv4 nor IPv6. So, if you don't get to control your IPv6
reverse PTR, you probably shouldn't be sending email from that
machine, because none of it is going to get through.

GMail certainly will never let you talk on IPv6 without a reverse PTR record.
I'm sure FreeBSD MX is just following "best" practices.

> For IPv6 this seems like a very strict requirement as it’s almost impossible to get reverse DNS working for autoconfigured IPv6 adresses ?

Mail servers typically have static IP addresses? Servers that aren't
mail servers but still send email should probably relay through a
fixed submission server.

> Is there an IPv4 only alias I can route mail to for  domain ?

You could always configure your email server to only be sending IPv4
email to certain domains. Ie for postfix, something like

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