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

I might conclude that the delivery agent triggers the bounce. But I've
never seen this defined as part of the Delivery Agent specification.

I'm eager to learn more about this, so links to additional
documentation will be very much appreciated.

Thanks,

Jim Schueler
Motor City Interactive