2.5. A Note on Controversies

A number of the described techniques are not universally accepted. For instance:

• Some argue that DNS Integrity Checks and DNS blocklists penalize individual mail senders purely based on their Internet Service Provider (ISP), not on the merits of their particular message.

• Some find that Sender Authorization Schemes like the Sender Policy Framework give ISPs a way to lock their customers in, and do not adequately address users who roam between different networks or who forward their e-mail from one host to another.

I will try to steer away from these controversies. Instead, I will try to provide a functional description of the various techniques available, including their possible side effects, and then talk a little about my own experiences using some of them.

That said, there are some filtering methods in use today that I deliberately omit from this document:

• Challenge/response systems (like TMDA). Not only do I think that these are fundamentally a bad idea[1]. The main reason I do not cover this method is that it is not suitable for SMTP transaction-time filtering. Instead, it relies on first accepting the mail mail, then returning a confirmation request to the Envelope Sender. This technique is therefore outside the scope of this document.

• Bayesian Filters. These require training specific to a particular user, and/or a particular language. As such, these too are not normally suitable for use during the SMTP transaction (But see User Settings and Data).

• Micropayment Schemes are not really suitable for weeding out junk mail until all the world's legitimate mail is sent with a virtual postage stamp. (Though in the mean time, they can be used for the opposite purpose - that is, to accept mail carrying the stamp, but that would otherwise be rejected).