Dear All,
the very original meaning of an "MX record" in DNS, is Mail Exchanger.
As such, in *absence* of SPF information in TXT of a given domain,
any single host defined as MX in a zonefile should be treated,
by default, as a valid sender AND receiver of email.
Such a filtering decision, would be beneficial to thousands
if not millions of domain owners by not blocking the hosts
that are obviously authoritative for sending emails.
On the contrary, by not doing so we will severely damage a great
number of small Internet users that don't have sysadmins or the
technical means to easily implement SPF (think of appliances).
PS.
In the future, we might end up with MS and MR records in DNS,
standing for Mail Senders and Mail Receivers respectively.
Until that day, we should consider MX as representing both,
rather cutting with a knife non SPF-adopters. If not, why?
all yours,
Fotis Georgatos
Network Engineer,
Secretary General of Hellenic Association of Internet Users