MX servers refusing mail from host via ipv6

Paul van der Zwan paulz42 at
Fri Oct 1 21:23:29 UTC 2021

> On 1 Oct 2021, at 22:41, Doug McIntyre <merlyn at> wrote:
> 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.
My own mailserver refuses to relay when no PTR is found but accepts mail destined for the local domain.
Which I considered an acceptable compromise.

> GMail certainly will never let you talk on IPv6 without a reverse PTR record.

Gmail has no problem whatsoever accepting mail from my mailserver.

> I'm sure FreeBSD MX is just following "best" practices.
This is the first time I ever ran into this so FreeBSD MX seems to be an exception.

>> 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
Having an ipv4 only MX would be nice but I guess I’ll have to figure out a local solution.
Running sendmail here so the postfix solution will not work but I think I can add an IPV4 only mailer and use mailertable to route domains like <> to that mailer.


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