Thanks Per.

I appreciate that mail operations should use an RFC 821 error message
instead of sending bounces. So I expected better documentation and
more interest in my post.

As soon as the Recipient is specified in the SMTP dialog, sendmail must
run through Ruleset 0 to determine by the mailer whether the w option
is invoked. If the fork and exec operations also occur at this point,
then a delivery agent exit code could trigger an RFC 821 message... if
it exits immediately. This solution is more practical than generating
the error within the rulesets.
However sendmail would have to handle an exit code differently
depending on when the agent exits.

Your response suggests otherwise. Is it worth the effort for me to
test my theory?

Thanks,
-Jim

Per Hedeland wrote:
> In article <1158674372.273496.75390@h48g2000cwc.googlegroups. com>
> "gymshoes" <jschueler@tqis.com> writes:
> >I've read the sendmail bat book, and I still don't understand how the
> >bounce mechanism works. It seems that the built-in error delivery
> >agent can be used to generate a bounce. But I don't see any reference
> >in sendmail.cf to the following message:
> > 550 5.1.1 foobar... User unknown
> >
> >Nor does Rule testing give the expected results:
> > [root@mail content]# /usr/sbin/sendmail -bt
> > ADDRESS TEST MODE (ruleset 3 NOT automatically invoked)
> > Enter <ruleset> <address>
> > > 3,0 foobar
> > ...
> > parse returns: $# local $: foobar
>
> This check (normally) happens internally in sendmail, based on the
> presence of the 'w' flag in the mailer definition - from doc/op/op.*:
>
> w The user must have a valid account on this
> machine, i.e., getpwnam must succeed. If not, the
> mail is bounced. See also the MailBoxDatabase
> option. This is required to get ".forward" capa-
> bility.
>
> Basically sendmail does a getpwnam() on the recipient name.
>
> >I might conclude that the delivery agent triggers the bounce. But I've
> >never seen this defined as part of the Delivery Agent specification.
>
> You've seen a Delivery Agent specification? Where?:-) Anyway, it's
> certainly conceivable for the delivery agent to generate and send a
> *bounce* of its own, after all it's just another e-mail message - though
> better would be for it to tell sendmail to do it by means of a non-zero
> exit code, e.g. for this case (from sysexits.h):
>
> #define EX_NOUSER 67 /* addressee unknown */
>
> However leaving the decision to the delivery agent (via either of the
> two alternatives above) is undesirable in the spam era, precisely
> because it entails the sending of a bounce - the standard method of
> having sendmail check internally, as well as any check implemented in
> the rulesets, has the advantage of causing a rejection of the recipient
> already in the SMTP dialogue.
>
> --Per Hedeland
> per@hedeland.org