Re: Re: [spf-help] SRS for Exim

On Wed, 28 Apr 2004, Bill Moseley wrote:

> I currently forward mail.  Does that mean that right now if someone
> sends mail from, say, AOL (that publishes SPF) to an address I host
> and I forward that mail on to a host that verifies SPF records that the
> mail will be rejected?

For your example of AOL, then no, AOL's SPF record never results in
FAIL.  You get PASS or NEUTRAL (or ERROR).  However, in the more general
case:

If you are forwarding for yourself, then no, not if you whitelist your
forwarding machine on the recipient system.  If you are forwarding for lots of
end users who don't know what SPF is and don't know how or why to whitelist
the forwarder, then yes, you need to rewrite the sender in case any of them
check SPF.

> BTW -- when forwarding can rewriting of the sender be skipped if the
> sender's domain doesn't publish SPF txt record?

Yes - although it's *much* simpler to just always rewrite - even when
not forwarding.  This allows you to reject those annoying bounces of
spam and worms you didn't send - plus those worms that try to "worm" their
way past spam defences by pretending to be a DSN.  If you always rewrite,
then any bounces (MAIL FROM: <>) that come to your machine are spams
and worms.  

In fact, always rewriting is often called SES - and if receivers
implement CBV, you don't need SPF.  However, SPF is better because 
TCP connections are comparatively expensive.  With only SES, anyone could
easily DOS your system by propagating lots of spam with your return address.
Millions of recipient systems calling your system to check whether the 
sender is valid is not something you want to suffer through.

(I had this happen to me.  My solution: I changed the MX of the domain
to point to some spammer machines.  Kind of drastic, but it worked.  No,
the domain was no longer used, so no legit mail got sent to the spammers.)


-- 
	      Stuart D. Gathman <[email protected]>
    Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
"Confutatis maledictis, flamis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.