On Jun 16, 2009, at 3:55 PM, John Levine wrote:
How do I find if I have blocked the domain from sending to my
server. Meaning, knowing the domain name of the sender, how do I
find the IPs from where the mail could be sent from. It seems that
SPF is the only tool to provide that answer?
Unless you have previous mail from the domain, I would agree SPF is
your best bet.
This is not your only bet. Many SPF records include the term MX and,
when not found, even default to using MX/24.
In another related problem, which is linked to IPv6 and RBL.
Buidling an IPv6 RBL could lead to a huge database. Sure you can
alleviate by using "wildcards", but why not use the reverse DNS
resolution to add a TXT record associated to the IP to indicate the
IP is the one of a mail server? So any IP that does not have this
record would be blocked for SMTP.
We've had a variety of proposals to identify mail client hosts. See
http://mipassoc.org/csv/
The CSV effort proved most providers do not want their MTAs identified
as belonging to them, even when it could improve email acceptance.
This might be especially true now after their support staff has been
reduced.
Reverse DNS is already causing a large amount of resources to be
wasted by the shabby state of the reverse name space. Incorrectly
configured RFC 2317 delegation, and many non-functional servers are
causing MTAs to rapidly become resource limited when making reverse
checks. In addition, when your customers conduct business with Asia,
they may not be happy to find email is being lost as a result of
geographic differences of opinion about the role that reverse DNS
might play with email.
IMHO, all outbound MTAs should be required to return CVS records for
their EHLO name and offer MX records for their inbound. A mandate
that required MX (inbound) or CVS (outbound) records would greatly
help in identifying non-abusive email sources against a backdrop of
hundreds of millions of bot-net controlled drones spewing email.
Systems may soon use ACLs as a means to white-list safe MTAs. Perhaps
the world is a few years from having to go to that extreme.
-Doug
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