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On Wed, Dec 15, 2004 at 11:35:36AM -0500, Dan Coutu wrote:

> So first let me ask, does sendmail actually work this
> way, with no limit on ping-pong bounces?

   What SHOULD happen is that the second bounce (the double-bounce)
gets sent to postmaster@ instead of the invalid user.  So in your case
it should look like:
   Spammer forges [EMAIL PROTECTED] to [EMAIL PROTECTED]
   random.foo bounces "User Unknown" to [EMAIL PROTECTED]
   client bounces the bounce to [EMAIL PROTECTED]

   Now, if someone's bouncing postmaster mail I'm not sure what will
happen. 


> If it does then there must certainly be a way to stop it, such as
> silently dropping failed mail.

   You can configure a custom delivery address for double-bounces.  In
some places I've used this in the sendmail.mc:

dnl # Drop all double-bounces.
dnl # Either supply an address (postmaster) or null string to drop.
define(`confDOUBLE_BOUNCE_ADDRESS', `')dnl

   I'm not sure what this translates to in .cf syntax.  It's something
frightening, I'm sure.

- -- 
Matt Brodeur                                                       RHCE
[EMAIL PROTECTED]                           http://www.NextTime.com

Hard work has a future payoff. Laziness pays off now. 
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