There are lots of machines which do not have the SMTP port configured
yet have A or AAAA records resulting in a implicit MX record and
week+ long no delivery notifications.
Just about everyone with a outsourced HTTP service needs to be able
to stop MTAs sending to email to the outsourced service. MUA's
could also lookup the MX RRset and issue a error without talking
to the MSA.
I can remember adding a null SMTP service back in the early 90's
that just 500'd all connection attempts to deal with all the miss
directed email accidently sent to desktop machines that I never
wanted to receive email even if they emitted it. This was much
better than having to chase down "why didn't xxxx get my email, I
sent it yyy days ago". The immediate bounce is important and getting
it a close to the sender as possible is important as it reduces the
chance that it will be dropped / missed.
The alternative to this is to remove the implicit MX record
construction from SMTP and make the presence of MX records manditory
for SMTP. I'm sure there will be many more complaints about doing
that than adding a explict no service record.
As for this along with other explict mx records. I would say to
ignore this record in the M[TU]A. Zone checking tools could warn
/ error if this is present. UPDATE could reject adding records if
there would be this and another record. Name servers could be
configured to warn / reject zones which have this and other MX
records.
--
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742 INTERNET: marka(_at_)isc(_dot_)org