Yes. Relays MUST simply relay messages. Only when a message
has reached an MTA under the *direct control of the recipient*
can other actions be taken. Is everybody happy with that
phrase before I start sprinkling it throughout the draft? :)
I don't mind the phrase, but you need to define it somewhere. For
instance, does "direct control of the recipient" allow an ISP to make
assertions on behalf of its customers? Does it allow a business's SMTP
server to make assertions on behalf of its employees?
Oof. No way am I going to try and define that concept. :)
How about a reference to section 7.7 of 2821, "Scope of
Operation of SMTP Servers"? That seems to cover this
situation pretty carefully and I can quote the whole
section (it only has 2 paragraphs).