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23 June 2004
This morning, I discovered that the /home partition on my mail server (a venerable 486 in our basement) was full, much to the consternation of qmail and courier-imap. I cleaned up my home directory on that server and everything seemed to be working again. My wife reported that she didn’t notice any problems with her own mail due to the problem, but she has had less spam than usual.
This gives me a wonderful idea for a spam-filtering technique: for 12 hours out of every 24, simply block all incoming email connections on your server. For legitimate users who send mail to a blocked machine, their ISPs’ mail servers will simply try again in a few hours, but the bulk-mail senders that spammers use probably won’t bother, because those programs are designed for people who want to fire off a few million messages and move on before they can be traced. Sure, any email that you care about receiving could be delayed for some random length of time, but that’s the price you pay for reducing incoming spam by half. Or you could block those connections for 18 hours out of every 24, and reduce incoming spam by two-thirds…
As D. J. Bernstein observes, one of the fundamental flaws in Internet email is that the recipient (or the recipient’s ISP) is responsible for storing his or her own mail. It would make much more sense for the sender (or the sender’s ISP) to take responsibility for storing messages and simply tell the recipient, “the next time you read your mail, there’s a message waiting for you to download at such-and-such a place”. If the Internet worked that way, then spam would be much less of a problem, because any message could be traced back to a server under the sender’s control with a permanent Internet connection, and the person owning that connection could be held accountable. And the odds of ever getting the whole Internet to make such a radical shift in the way it uses email are … well, about equal to the odds of getting all the spammers to give up their trade out of the goodness of their hearts.