A new ESMTP extension DATA-CALLBACK.
The current DATA phase becomes a the header only portion of the
message. A new header also provides a token used by the MUA to
retrieve the rest of the data.
I like this, for normal mail transport. I don't see it as being of any
particular anti-spam use, though; spam will simply be sent without
using it, the way it is today. (Given all the usual deployment issues,
it'll be a long time before anyone but the most lunatic fringe can
afford to reject mail not sent using it.)
A suggestion has been made before (here I think defintely elsewhere)
that a temporary error response might be issued after RCPT (indeed
after any command) and before DATA which would allow the recipient
MTA to carry out asynchronous verification of the sender MTA and any
other checks deemed necessary without the cost of recieveing the
message content.
Except for the bit about putting a minimum delay on the retry, this
amounts to greylisting. The hardest part here is telling whether a new
SMTP session is a retry of a previously-rejected one or a new message.
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