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Re: Why we should choose the RFC2821 MAIL FROM/HELO identities

2004-04-06 08:17:10

Philip Miller <millenix(_at_)zemos(_dot_)net> wrote:
The messages must get delivered, or bounced, or dropped. Inevitably, some
ISPs will not bounce mail that doesn't validate, but simply throw it away.
We can say that's a bad thing for them to do, but we can't stop them from
doing it.

  That's true not only because of ISP's bad decisions, but because of
flaws in SMTP.  RFC 2821 says an MTA must accept a message for
delivery or bounce it.  This says to me that an MTA should verify the
bounce path before accepting a message for delivery, as there may be
later delivery failures.

  However, RFC 2821 makes no provision for verifying the bounce path.
So in many cases, an MTA has no choice but to discard an undeliverable
message.  the MAIL FROM validation checks can be viewed as an attempt
to ensure that a bounce path exists.

  Alan DeKok.