Keith Moore writes:
IMO, With a few notable exceptions (e.g. submission servers, spam
filters, gateways to foreign systems), MTAs shouldn't be checking
message headers at all.
AFAICR (right now) all the MTAs I've heard of have at least one of those
features, most have two, and are covered by these exceptions.
Makes the rule fairly irrelevant.
The problem is that a message typically has to pass through several MTAs.
... [much elided] ...
Yes, all very sensible, but in the end, there always are spam filters
and often exchange gateways and other exempted hops, so the actions of
any other MTAs are essentially insignificant.
Arnt