At 11:21 28-11-2008, David Nicol wrote:
On Fri, Nov 28, 2008 at 9:06 AM, John Levine <johnl(_at_)taugh(_dot_)com> wrote:
> Also, SMTP is store-and-forward and [...] multiple hops.
It was designed that way, for UUCP bang-paths, but in practice for a
very long time now, especially since running open relays began being
poor form, SMTP has been sender-realm systems communicating directly
to recipient-realm systems.
When I send out this message, it is submitted to a MSA where it is
queued. The message is then relayed to the MX host for irtf.org
where it gets delivered to the recipient. Section 3.9.2 of RFC 5321
explains how the message gets redistributed to the subscribers of
this mailing list. There is some store and forward being done. The
SMTP server cannot take full responsibility for a message without doing that.
There have been several proposals about moving away from the
store-and-forward model. Some of these proposals assume that there
is direct connectivity between the sending and receiving
systems. That's not the case. A message can go through several hops
before being delivered to the recipient. It's not uncommon to see
that nowadays when mail filtering has to be performed. Although
there isn't a lot of source routing being done, people still use
routing tricks even though it's not transparent to the sender.
Regards,
-sm
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