"LISTSERV site administrators' forum" <[log in to unmask]> 
wrote on 11/01/2006 01:03:09 PM:

> It can be configured not to return bounces.  For non-listserv 
> installations, returning bounces does perform the useful task of 
> informing legitimate people that their email did not get through and 
> they can then take appropriate action.  It's possible to mount a 
> reverse attack, i.e. have some poor unsuspecting individual get 
> thousands of real bounces because their email address was used in one 
> or more large spam mailings.

Returning bogus mail is just plain wrong.  The trick is to *NOT* accept 
it.  This requires evaluating messages DURING the connection, before 
returning "250 message accepted".  Any of the spam filters based on 
sendmail's milter interface can do this.  Other MTAs may be able to do 
this as well.  Closing the connection and then evaluation is going to 
guarantee either lost mail or backscatter spam.