> > Accepting messages from a domain lacking MX records might be risky
> > due to the high rate of domain turnovers. Within a few weeks,
> > more than the number of existing domains will have been added and
> > deleted by then. Spammers take advantage of this flux. SMTP
> > server discovery via A records is permitted and should be
> > deprecated.
> All it would require is a couple of large ISP's to adopt
> such a policy. "MX 0 <self>" really is not hard and benefits
> the remote caches.
Agreed. While some suggest deprecating A record discovery requires
adoption by a standards body, it really only requires a few ISPs to make
their intentions public. A small minority of domains lacking an MX
record are likely to comply quickly. At that point, adoption by a
standards body becomes possible. It is rare to find a standards body
willing impose additional requirements on email, but this is a case
where such a requirement is clearly necessary.
That point forward, spammers would be less able to take advantage
of domains in flux, and policy schemes would be far less perilous for
roots or second level domains.
> > Once MX records are adopted as an _acceptance_
> > requisite, domains not intended to receive or send email would be
> > clearly denoted by the absence of MX records. SMTP policy
> > published adjacent to MX records also eliminates a need for email
> > policy "discovery" as well. Another looming problem.
> Better yet use MX records to signal that you don't want to
> receive email e.g. "MX 0 .". It has a additional benefits
> in that it is *much* smaller to cache than a negative
> response. It's also smaller to cache than a A record.
> Since all valid email domains are required to have a working
> postmaster you can safely drop any email from such domains.
Use of root "." as a name for a target may create undesired non-cached
traffic when applications unaware of this convention then attempt to
resolve an address for servers named root.
All modern iterative resolvers are required to support
The use of root as a convention will complicate a general strategy
identifying adoption of a protocol by publication of a discovery
record. The use of root as a target name in SRV records has been
problematic, although this convention was defined for SRV records at the
Using an MX record to mean "no email is accepted" by naming the
target 'root' changes the meaning of the MX record.
Not really. It's entirely consistant with existing DNS
usage where "." is a domain name / hostname place holder.
Lots of RR types use "." to indicate non-existance.
It is also not clear
whether the root target would mean "no email is sent" as well.
That is, I'll agree, more of a issue but no one can reasonably
expect people to accept non-repliable email.
A clearer and safer strategy would be to insist that anyone who cares
about their email delivery, publish a valid MX record. Especially when
the domain is that of a government agency dealing with emergencies. At
least FEMA now publishes an MX record. This requirement should have
been imposed long ago. : )
I much prefer positive data vs the absence of data to make a
decision. "MX 0 ." is a definative response saying you don't