The question came offlist. But it is a good question, central to the
issue.
You can lookup an MX record, or given only an A record, you can try to
send mail to that address. Only the latter will give you any clue as to
whether you can send mail to that address. And unless you implement
something like qsecretary, even that will not truly indicate whether mail
can be returned to the sender.
This is the fundamental flaw in DNS based "spam checking". DNS cannot tell
you whether a particular sender is a spammer. Nor can it tell you if the
From: address is a valid address, or if the mail truly originated from
that person. Assuming that it can, leads to trouble, from which there is
no escape.
Content analysis is your friend for spam detection. DNS checks aren't.
--Dean
---------- Forwarded message ----------
On Thu, Sep 18, 2003 at 06:17:21PM -0400, Dean Anderson wrote:
DNS, Zone files, etc, are not acceptable means to query the set.
[..]
Dean,
Sometimes one doesn't want to know if a domain is *registered*, one
simply wants to know whether mail can be sent to it.
Given that it is most likely impossible to achieve a certain answer (and
WHOIS definitely doesn't help one jot!), what do you propose is a decent
and accepted method of achieving a best guess?