Non content based antispam sucks

My provider has recently changed the IP address of one of my server and my logs are flooded with messages such as:

Dec  7 08:21:57 gwnormandy postfix/smtp[22362]: connect to mx00.schlund.de[212.227.15.134]: server refused to talk to me: 421 Mails from this IP temporarily refused: Dynamic IP Addresses See: http://www.sorbs.net/lookup.shtml?213.41.184.90   (port 25)
Dec  7 08:21:57 gwnormandy postfix/smtp[22339]: connect to mx01.schlund.de[212.227.15.150]: server refused to talk to me: 421 Mails from this IP temporarily refused: Dynamic IP Addresses See: http://www.sorbs.net/lookup.shtml?213.41.184.90   (port 25)
Dec  7 08:21:57 gwnormandy postfix/smtp[22334]: connect to mx01.kundenserver.de[212.227.15.150]: server refused to talk to me: 421 Mails from this IP temporarily refused: Dynamic IP Addresses See: http://www.sorbs.net/lookup.shtml?213.41.184.90   (port 25)
Dec  7 08:21:57 gwnormandy postfix/smtp[22414]: connect to mx00.1and1.com[217.160.230.12]: server refused to talk to me: 421 Mails from this IP temporarily refused: Dynamic IP Addresses See: http://www.sorbs.net/lookup.shtml?213.41.184.90   (port 25)

Of course, I am trying to get this solved by sorbs.net (in that case, that should be possible since this is a fixed IP) but that incident reminds me why I think that we shouldn’t use « technical » or « non content based » antispam even if it happens to be efficient.

The basic idea of most if not all antispam software is to distinguish between what looks like a spam and what looks like a normal message.

To implement this, we’ve got three main types of implementations that can be combined:

• Content based algorithms look at the content of the messages and use statistical methods to distinguish between « spam » and « ham » (non spam).
• List based algorithms work with white and black lists to allow or deny mails, usually based on the address of mails sender.
• Technical based algorithms look at the mail headers to reject most common practises used by spammers.

The problem with these technical algorithms is that the common practises used by spammers are not always practises that are not standard compliant and not even practises that should be considered as bad practises!

Let’s take the case of the sorbs.net database that identify dynamic IP addresses.

I would argue that sending a mail from a dynamic IP address is a good practise and that asking people to use their ISP mail servers when they don’t want to is a bad practise.

I personally consider that my mail is too important and sensitive for me to be outsourced to my ISP!

That’s the case when I am at home and I prefer to set up my own smtp servers that will take care of delivering my mails than using the smtp servers from my ISP.

When I am using my servers, I know from my logs if and when the smtp server of my recipients receive and queue the mails I am sending.

Also, I want to be able to manage mailing lists without having to ask to anyone.

And that’s still more the case when I am travelling and using an occasional ISP that I barely know and don’t know if I can trust.

We are using lots of these ISP when we are connected to WIFI spots and here again, I much prefer to send my mails from the smtp server that runs on my portable than from an unknown ISP.

Furthermore, that means that I don’t have to change the configuration of my mailer.

Content based antispam have also their flaws (they need training and are very inefficient with mails containing only pictures) but they don’t have false positives like technical based antispams that reject my mails if I send them from dynamic IP addresses.

That’s the reason why I have desinstalled Spam Assassin and replaced if with SpamBayes on my own systems.

Now, the thing that really puzzles me with antispam is that we have the technical solution that could eradicate spam from the web and that we just seem to ignore it.

If everyone was signing his mails with a PGP key, I could reject (or moderate) all the emails which are not signed.

Spammers would have to choose between signing their mails and being identified (meaning they could be sued) or not signing them and getting their mails trashed.

Now, the problem is that because so few people are signing their mails, I can’t afford to ignore unsigned mails and because PGP signatures are not handled correctly by many mailers and mailing list servers, most people (including me) don’t sign their mails.

The question is why doesn’t that change? Is this just a question of usages? Or is the community as a whole just not motivated to shut the spam down?