One growing technique for use in anti-spam involves finding ways to “fail” on initial contacts for sending mail. Real, standard conformant mail programs try again in various ways, but spammers, in writing their mail blasters, tend to just have them skip that address and go to the next one in their list.
Two common approaches include simply returning a “temporarily unavailable” status on any initial mail attempt that might be spam. Another approach is to have dead MX records both at the “try first” and “try last” end of the MX chain.
Why does this work? Spammers just want to deliver as much mail as possible given time and bandwidth. If one address fails for any reason, it’s really no different whether you spend your resources trying the address again or in a different way, or just move on to the next address. In fact, since many of the failures are real failures, it’s actually more productive to just move on.
And, I admit, some of the spam filtering tools I make use of use these techniques, and they do help. But what exactly are they doing? For spammers, the limiting factor is bandwidth. Dealing with failures, especially timeouts on dead servers, takes very little of their resources.
It doesn’t reduce the amount of spam they send, at least by much, it just redistributes it to those who don’t use the techniques. For a positive spin, you can liken it to putting up a higher fence than your neighbour, so the criminals attack them and not you. For a negative spin, you can imagine it as being like an air filter that filters out the pollution on air coming into your house, and spews it out the back at your neighbours.
So it’s a touch question. Is this approach a good idea? Especially at the start, it was very effective. Over time if it becomes very common spammers will see a reduction in spam they deliver and make fairly simple moves to compensate for it. Is this fair game or antisocial?
There is an old joke about two hikers who meet a bear. The first sits down and starts putting on his running shoes. The other says, “What are you doing, you can’t outrun a bear!” and the first says, “I don’t have to outrun the bear, I just have to outrun you.”
Are we passing the bear onto our neighbours?
(This is part of a larger question of some of the other negative consequences of anti-spam. For example, as text filters got better, spammers moved to sending their spam as embedded images which filters could not easily decode. The result is more and more bandwidth used, both by spammers and victims. Was it a victory or a loss?)