[aklug] SMTP: Why the middle-man?

Okay, perhaps this is just an academic question now, but has anyone ever
wondered why, in the paradigm of e-mail communications, there is always
a "man in the middle" (MSA) for the submission aspect?
It makes sense that e-mail end-users cannot receive their own mail
directly, as there aren't enough static IP addresses and
easy-to-remember domain names to go around. Plus, it makes sense for
multiple users to share a single mail exchanger, which is separately
dedicated to doing things like filtering out spam.

But on the /sending/ side of e-mail communications: why don't we all
just use SMTP to submit our e-mails directly to the mail exchanger for
each destination? (As opposed to our current system, where we transmit
the e-mail to an MSA, which then relays it to to the MX.)

It would make e-mail client configuration slightly easier (no
server/authentication information required for e-mail submission). The
e-mail client could just use DNS to figure out where to connect.

Probably the answer I'm going to get is "well, then we couldn't have
PBLs, and spamming would get out of control". But is this really the
case? For starters, have PBLs really stopped spamming anyway? Besides,
spamming is done through HTTP all the time (blog comment spamming, for
example) but we don't force Web surfers to use "Web Submission Agents".
Furthermore, it seems to me like the MXs themselves should be basing
their filtering on the content of the received e-mails (spam filtering
based on from addresses, Subject line keywords, body text analysis,
well-formed headers, and so forth) rather than the IP address of the
last point in the relay.

Penny for your thoughts. (Actually, I don't have a penny...)

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