When are anti-spam techniques not actually helpful?

By Bruce Johnson
4 November 2003

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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 won’t 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 I’d write it off as a dumb user, but it seems to be more and more the policy of ISP’s to do this. (Here’s the irony – it forces us to redirect our email through the ISP’s 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.)

What’s particularly galling is that this technique has absolutely no visible effect on the amount of Spam whatsoever. Because it’s relatively easy to bypass this check it doesn’t stop any real Spam, but it does an excellent job of filtering out mail you probably want.

This isn’t 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 it’s 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.

More inconvenience.

And spam is completely unaffected.

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