== An Internet rule of thumb

> Any Internet protocol change that requires everyone's participation is
> dead on arrival.

This often applies to would-be antispam efforts such as [[SPF
http://en.wikipedia.org/wiki/Sender_Policy_Framework]].

(Superficially, [[SPF]] looks like it is an isolated thing that only
involves the sender having SPF records and the receiver checking
them. It's not, and the way it requires everyone's participation
is illustrated by the need for the [[Sender Rewriting Scheme
http://en.wikipedia.org/wiki/Sender_Rewriting_Scheme]].)