I question the reason for the difference, given that the current MTA
".forward" mechanism and aliasing appear to stand in the way of
allowing a straightforward SPF implementation, particularly because
the "original author" can be foreign to the MTA doing the
forwarding.
I suggest that you have your causal concerns, or power relationships,
reversed.
The problem is that SPF imposes an unreasonable restriction on
long-established, highly reasonable uses of email.
I submit that neither view is particularly helpful to reaching any sort
of workable resolution.
There is an incompatability between SPF and .forward-style forwarding;
I think everyone agrees on that. SPF proponents see this as a problem
with .forward; opponents, a problem with SPF. Neither view is
automatically "right".
But I'll suggest that none of this matters. The .forward facility is
a) a good idea, and b) entirely legal within all current standards.
The same could have been said of open relays, when they first began to
be a problem.
This is not to say that I want to argue on either side. I simply find
this line of argument unconvincing; I am presenting an opposing point
not because I oppose the conclusion but because I oppose the argument.
In any event, it is always a bit challenging to change a
long-established practice for a service in use by a billion people...
True. I think the question we need to be asking is "is this, however
widespread and however longstanding, something we will ultimately be
forced to give up?". If the answer is "no", we should try to find
antispam tactics that preserve it. If the answer is "yes", we should
mentally give up on it now and figure out how to best manage the
(inevitable, in that case) transition.
I'm not convinced of either "yes" or "no" at this point.
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