Re: [exim] HELO/EHLO reject rates

Interesting observation. Unfortunately I don't keep historical data for
individual rejection reasons (possibly I should), but my feeling (and
it's only that) is that there has been an increase in the use of domain
literals as HELO/EHLO strings. Although (AFAIK) these are perfectly
legal, we now reject mail where the HELO/EHLO string is a domain literal
of the sending IP address AND there is no rDNS for the sending IP
address.

We also reject where the HELO/EHLO string is a single word (i.e. no "."
in it, so it can't be a FQDN or domain literal) AND there is no rDNS on
the sending IP.

No complaints about either of these so far (they probably count for
upwards of 5% of all rejections, despite being fairly late in the
sequence of tests).

I'd love to reject wherever there is no rDNS, but I think there would be
too many false positives involved. (I know that some here take the view
that this is not a false positive, but our users are likely to regard a
message that is not spam, and does not originate from a known source of
spam, as one that should be delivered). That's not to say it can't be
given a score in SpamAssassin though.

On a slightly related issue - I have an idea that the hit rate from RBLs
(we prinicpally use MAPS+ and Spamhaus) may not be as high is it was a
couple of months ago. Does anyone else have the same feeling (or any
data to confirm/deny)?

Regards

Richard

--
Richard Rogers
IT Development and Innovation Manager
Information Services, Staffordshire University


> -----Original Message-----
> From: exim-users-bounces@???
> [mailto:exim-users-bounces@exim.org] On Behalf Of Phil Pennock
> Sent: 19 September 2007 10:32
> To: exim-users@???
> Subject: [exim] HELO/EHLO reject rates
>
> It appears that the effectiveness of filtering out known-bad HELO/EHLO
> has dropped somewhat in the past few months:
>
>
> http://people.spodhuis.org/phil.pennock/img/exim-reject.2007-09-19.png
>
> http://people.spodhuis.org/phil.pennock/img/exim-reject.2007-0
> 9-19.ylog.png
>
> Of course, this is in absolute numbers rather than a rate of HELOs
> received so it could be a lull in the connection attempts
> overall, but I
> doubt it, especially given the recent issues people have seen with
> parallel connections for major pumping.
>
> The y-axis is how many SMTP connections have been rejected, per day,
> based on this HELO/EHLO string; the normal IP in the legend is my
> system's IPv4 address -- I don't like remote people sending
> me my own IP
> address in HELO.
>
> This system is for a private colocation host handling a few personal
> domains with a very few non-local users (local user count is 2).
> Between 130 and 400 mails per day are actually delivered, mostly spam
> into spam-folders.
>
> -Phil
>
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