When are anti-spam techniques
not actually helpful?
By Bruce Johnson
4 November 2003
We recently had cause to rearrange our email servers. Almost immediately we started
getting mail bouncing because some servers considered our server to be using spam.
Forgive me if I get a bit technical here, but essentially the other server was
doing a Reverse-DNS lookup. This basically means that when our server wants to
send a mail it naturally announces who it is. (In our case our domain, which is
capesoft.com). The server at the other end does a quick lookup to see if it was
to send us email, if it would go to the same machine. Since it wont it complains,
and bounces the mail.
Now this seems fair, until you consider that nowhere in the email specification
is it even suggested that you should run your sending-email-server, and receiving-email-server
on the same machine. In fact after a brief understanding it becomes clear that
they can be on different machines, and most often (for performance reasons) are.
So here we have perfectly legitimate mail being bounced because of an overzealous
server. If it was an isolated case then Id write it off as a dumb user,
but it seems to be more and more the policy of ISPs to do this. (Heres
the irony it forces us to redirect our email through the ISPs mail
server, using up their resources, and slowing down our mail. So not only does
it have no positive effect, but ultimately makes the email slower.)
Whats particularly galling is that this technique has absolutely no visible
effect on the amount of Spam whatsoever. Because its relatively easy to
bypass this check it doesnt stop any real Spam, but it does an excellent
job of filtering out mail you probably want.
This isnt the first time this has happened. A few years ago a lot of ISP
decided to cap the number of emails that could be sent on a single connection.
The number 50 seemed popular for some reason. One day our mailing list (running
from inside Outlook Express) simply failed. This approach for reducing spam is
so dumb that its actually noteworthy. Of course it took no time at all for
all the spamming software to change so that emails were sent a few at a time.
This has a minimal, to nonexistent, impact on Spam. But it took ages for operating
systems, and mail clients to catch up. The simple user suffers.
Another common tactic is for the ISP to blacklist their own IP numbers with other
ISP's so as to prevent their own customers from sending mail directly.
A case in point is where you try and send an email directly to an AOL recipient,
bypassing your own ISP's mail server. This is more efficient, lowers the burden
on your ISP, and makes the mail get there quicker.
But many ISP's are now reporting all their Dial-Up IP numbers as "blacklisted"
numbers. i.e. Your ISP goes to AOL and blacklists _you_. The ISP insists that you
route any mail via their own mailserver rather than send it directly.
And spam is completely unaffected.
: This article is the opinion of the author, and is not necessarily
the opinion of any other person or company.
Specifically it may not be the opinion of the owners, or host, of this site.
© 2012 CapeSoft Software