Re: [vps-mail] bouncebacks going to postmaster instead of sender

[Godwin Stewart <gstewart@xxxxxxxxxxx>]

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On Tue, 6 Jun 2006 10:00:41 -0600, Scott Wiersdorf <scott@xxxxxxxxxxxx>
wrote:

> For those on the list not familiar with the terminology, a double-
> bounce is one where the bounce address failed. For example,
> joe@yourserver send a message to someone@yahoo. If 'someone' doesn't
> exist at yahoo, yahoo will generate a bounce message to joe@yourserver
> indicating such (assuming that joe@yourserver is set in the
> 'Return-Path:' of the original message).

Sorry to nitpick but this isn't exactly how it works.

joe@yourserver sends a message to someone@yahoo. If Yahoo's MX rejects
the mail (no bounce is involved here, bounce != SMTP reject) instead of
accepting it, then the sendmail running on "yourserver" has to inform
joe@yourserver of the problem. However, if sendmail can't deliver the
DSN to joe@yourserver, then this is when the "double bounce" situation
arises. In this case, it informs "DoubleBounceAddress" that the DSN
intended for joe@yourserver was undeliverable.

There is a certain amount of confusion in the SMTP world. What is *now*
known as a bounce is as follows. Assume joe@yourserver sends the mail
to someone@yahoo. Yahoo *accepts* the mail, only to realize later on
that it can't be delivered. Yahoo then has to generate a new mail with
a standard warning text and send it off to the original sender of the
mail that it couldn't deal with, joe@yourserver. This mail from *Yahoo*
stating that the message couldn't be delivered is a "bounce".

A reject, OTOH, occurs if Yahoo already knows that mail sent to
someone@yahoo isn't going to be deliverable (because there's no such
account, for example). Instead of accepting joe's message and then
bouncing it, Yahoo replies to yourserver's request to send the mail
with "550 ptooey, I don't want this". In reaction to this, *yourserver*
sends a DSN to joe@yourserver informing him of the problem.

So, a "bounce" comes from the recipient's ISP while a "reject"
notification comes from the sender's ISP.

This distinction is crucial in dealing with spam. We all know that 99%
of the time, the addresses "From " which spam is sent are bogus. MX
servers that accept all mail and then *bounce* that which they can't
deliver are going to be sending the bounces to these bogus addresses,
which often belong to innocent people whose e-mail addresses were
harvested by spammers, or to spamtraps...

Reject == good, bounce == bad.

The term "double bounce" used with sendmail was coined in a long-gone
era when spam didn't account for 90+% of e-mail. In those days,
bouncing an undeliverable was the courteous thing to do. Nowadays it's
a good way to get yourself blocklisted :)

- -- 
G. Stewart - gstewart@xxxxxxxxxxx

Why is it that people say they slept like a baby when babies wake up
every two hours?
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