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SubjectRe: Stupid anti-spam testings...
[Philip Hazel bcced]
[Blue-labs postmaster bcced too]

From: Matti Aarnio <>
> On Mon, Sep 02, 2002 at 04:28:37PM -0600, Andreas Dilger wrote:
> ...

Hi folks. Since I've been involved in code in exim, spamassassin, and was
the postmaster for, I thought I could give some info on all this.

> > > Folks, when you deploy that kind of testers, DO VERIFY THAT THEY
> > > HAVE SANE CACHES! A positive result shall be cached for at least
> > > two hours, a negative result shall be cached for at least 30 minutes.
> >
> > Do you know if this is one of the default checks from spamassassin?

There is no code in spamassassin that does SMTP callbacks and I would
advocate against it if someone were to think about adding it. Callbacks need
to be done once, at SMTP time, not later for each receipient when it's
already too late to bounce the mail if the env from is non reachable.

> No idea. I have seen these coming from Exim 4.10, Exim-something,
> some sendmail milter (whatever that is), etc..

So, exim, has since version 3.2x (I think, might be 3.3x) something called
SMTP callbacks (now SMTP callouts).
It's an optional feature where the server does a NULL SMTP connection back
to the envelope and header sender to make sure that the servers would be
willing to accept mail back for those.

Goals are:
1) do not accept mail that you cannot bounce (i.e. limit dead mail/double
bounces in your spool)
2) make sure that mail you accept can be answered (bad header from)
3) Tell people that their envelope is broken (you can only do that by
rejecting the SMTP connection)
4) Refuse mail from morons who refuse null MAIL FROMs
5) and that's only a byproduct: refuse some junk mail/spam

If you want to know how it works in details, I've explained it here:

I think someone may eventually have written an SMTP milter to do the same
(I suggested it to Eric Allman when he talked at SVLUG last year)

> Apparently the idea (which I have thought of long ago, and rejected
> as incomplete) has caught, and has multiple implementations...

Not too many yet, and it is turned off by default on exim, however many are
turning it on (for better or for worse)

> I can easily reduce the load impact it causes to vger by
> running the smtp server in "accept everything" mode without
> analyzing local addresses for existence. With a bit more
> work I can throw in local cache.. (which I probably have to do..)

Hopefully it won't come to that.

> > If it is possible to track what tool is causing the problem and fixing
> > the default setup of that tool at the source, it will probably solve
> > 99% of the problems in one go (after the list knows to which version
> > they should upgrade).

As soon as Philip Hazel initially came up with the first implementation I
know in exim, I suggested to him that he should _really_ implement a
positive and negative cache.
Worse, if the header and envelope from are the same, the current code will
still open two SMTP connections back to check them both, even though they
are equal

Unfortunately Philip hasn't had the time to implement this yet, and others
(like me) haven't had the time to do it either and contribute code back.

> Raise some noise all around, there are multiple implementations
> of the idea. Some even with syntactically invalid tester codes
> (spaces put in place where they don't belong in RFC 821/2821);
> "works with sendmail" is NOT synonymous to "is syntactically
> correct."

I only know of the exim implementation, and I believe that one is correct.

> - a mister at runs some sendmail-milter which
> does testing with invalid protocol syntax

I think I know who it is, he may have written it after he saw the code I put
on, or he may have gotten it from somewhere else, I'll let him know.

> - use probably their own code
> too... possibly more systems there..

I don't remember the network layout, and it may have changed since I left,
but mail goes to one host which then dispatches it to sf lists or some alias.
I did change the code a little bit on to do an additional
postmaster callback because we got tired of having to support people who
filled our spool with crap and didn't accept Email for postmaster when we
tried to contact them and work things out.
The code uses the existing SMTP connection, so it issues an extra RCPT, but
that's very little extra load.

Note too that users aren't really supposed to subscribe to
mailing lists with those aliases. The alias is simply a
redirector you can use and advertize for your projects.
Now in practise, some users are apparently subscribing to mailing
lists anyway, but the mail server wasn't designed with that in mind.

Please note too that I am not employed by VA Software anymore, so you can
flame me, but I won't be able to do much to fix it. I know the
implementation is imperfect, I was only postmaster for on my "spare
time" since no one else was there to do it, but it was the best we could do
to handle junk mail, bad bounces, and spam send to several hundred thousand
users all running on mail server, and with no full time mail admin.
I know some things that can be done, I'll list a few here, but I never got
to upgrade to exim 4 with a callback exclude setup (doable with exim 3 too)
because I was layed off a couple of weeks before I'd have rolled out the

To contact the team, you can submit a support request here:
If you are not satisfied, you can also contact the site director at this

> - tests from Exim 4.10
> - tests from Exim 3.35

I'm surprized you're not seeing more exim users.

> Right now something like 5-7 different systems are doing it.
> Try to imagine when all 3500 targets do it... BRRRRR...
> (Sure, VGER can handle it, no problem, but it is that much
> wasted cycles, and network traffic...)

Ok, so here's my opinion on this since I studied callbacks extensively
before rolling them out on and hacking the code:

1) we've had 3 or 4 complaints in 2 years, mostly from people who didn't
like the extra lines in their mail logs
2) The SMTP callback does take a few resources, but they are rather minimal.
If you send lots of mail, you typically have the resources to handle
callbacks from remote sites.
3) Yes, it can get bad for a list server when many receipients start doing

What should be done about this:

1) Positive caching with a cache of at least a day, if not a week, should be
in every SMTP callback implementation

2) I'm looking at setting up a DNS list of list servers so that people don't
do callbacks on them. With exim 4, you could then query that list and not
do callbacks if the sending list server's IP is in that list.
Something can probably be done to exclude envelope from domains too.
If you are willing to help me with this, let me know

3) A 3rd, and lesser solution, is to agree on a mail header that tells
receiving servers not to do callbacks.
a) can obviously be faked by spammers, but I still think it'd work for
quite a while: spammers who are smart enough already fake valid
envelope and header froms, so they wouldn't have to add the header
b) exim can trivially add a header on outgoing mail and check for it
before doing callbacks on the receiving side, but I don't know how
trivial it is for other MTAs of sending list servers to add the

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Microsoft is to operating systems & security ....
.... what McDonalds is to gourmet cooking
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