We need to make the solution to the forwarding problem as unintrusive as
possible for the forwarding hosts.
SMTP already has a potential answer -- the 551 result code which gives
an alternative address to use instead of the address which was given in
the RCPT TO: command.
I think the SPF specification should make it clear that a forwarding
host, upon receiving mail from a domain with an SPF record, MAY return a
551 result code with the forwarding address.
And of course that the SMTP client responsible for _sending_ such mail
MUST deal appropriately with such responses, by delivering the mail in
question to the newly-provided forwarding address.
--
dwmw2