On Sat, Feb 01, 2025 at 04:11:48PM +0800, Philip Paeps via mailop wrote:
> > I'm afraid that sending email from a NullMX domain that does not accept
> > any bounces, replies, postmaster queries, ... is a lost cause. Plenty
> > of systems will reject attemtps to send mail from such a domain, mine
> > included, and I, for one, have not intention of changing that.
>
> One additional observation to note here: some places also reject mail from
> individual hosts that have a nullMX record, even if the domain has no nullMX
> record.
>
> E.g. an installation with separate servers for inbound and outbound email.
> The outbound server has proper FcrDNS and follows all other best practices.
> If the outbound server has a nullMX record, some sites will block it.
>
> example.net. MX 10 mail-in.example.net.
> mail-in.example.net. A 192.0.2.25
> mail-in.example.net. AAAA 2001:db8::25
> example.net. TXT "v=spf1 ip4:192.0.2.26 ip6:2001:db8::26 ~all"
> mail-out.example.net. A 192.0.2.26
> mail-out.example.net. AAAA 2001:db8::26
> mail-out.example.net. MX 0 .
>
> Even if the mail-in.example.net is happy to accept bounces, replies,
> postmaster queries etc. And mail-out.example.net never appears in an
> envelope or anywhere else that would solicit email: some sites will block
> mail from mail-out.example.net.
>
> If you want to be able to send mail, don't say you're not receiving mail. :)
> It's also a good idea to make sure your mail-out.example.net listens on port
> 25 ... some very naive places try to connect and get cranky if they can't
> open a socket.
>
> I discovered this in a setup that templates "MX 0 ." on every host that is
> not expected to receive mail.
Those receiving systems are broken, and should be ignored/punished for
their misconfiguration. It would be great if more operators published
NullMX for the individual MX hosts that are not also valid email
recipient domains.
--
Viktor.
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