As we are new to the IETF process, we welcome any suggestions
pointing out potential improvements as well as deficiencies in the draft.
Can you describe how this is superior to "greylisting", which, as I
understand it amounts to refusing mail from unclassified sources with a
"try again later" (TEMPFAIL) code the first time it is offered, but
accepting it if offered again several hours later?
Could greylisting be made even more like your plan if the receiving MTA
only accepted mail on the second try if it came from the same IP address?
The overall effect - requiring the sender to maintain their server for
several hours before mail is accepted - seems to be about the same. The
advantage for greylisting is that most legitimate senders are already able
to handle receivers using this technique, whereas DMTP would require an
upgrade to all senders to comply. I observe that MTA vendors and FOSS
sources of MTAs are extraordinarily resistant to new procedures.
I understand that greylisting works now because spammers haven't found it
worthwhile to record success or failure, and don't come back after a
TEMPFAIL. Of course, if many receiving MTAs started to use the technique,
spammers might respond by keeping such records. In that case, DMTP (or
greylisting, for that matter) would be effective only insofar as
blacklists could be expected to pick up new spammer addresses in a matter
of hours. Is that feasible? My impression is that while it may be
feasible, it is made necessary only by the tendency of DNSBLs to drop
entries rather quickly. If blacklist entries for dynamic addresses are
made very persistent, it isn't necessary to pick them up quickly, they
will already be on the list from the last spam run days or weeks before,
in which case the DNSBL itself is sufficient, and DMTP or greylisting is
not much additional help.
Daniel Feenberg
feenberg isat nber dotte org
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