Douglas Otis <dotis(_at_)mail-abuse(_dot_)org> wrote on 05/06/2006 22:45:53:
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. The recipient could check the DKIM
signatures before deciding whether the signing domain had a suitable
reputation before even requesting the information. The bulk of the
message would be held by the sender rather than by the intermediary
for the recipient.
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.
In the existing protocol this will be followed eventually by a re-try which
can then be accepted or rejected permanently based upon the new knowedge we
have gained about the sender address and sending system. An enhancement to
the error codes could include a retry interval, hinting to the sender the
optimum wait for the best balance between futile retries and undue delay.
An alternative would be that the recipient MTA could issue an ETRN to the
sender to hint to the sender that it could send any outstanding messages to
this recipient.
An enhancement of (or replacement for) ETRN might make it mean that the
sender should send any mail _from_ a particular _sender_ which is due to be
routed through this host, as this is possibly a more logical grouping if we
assume that the command is going to be used to "unblock" temporary
restrictions on individual senders.
d.
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