Bounce problems and spam filters?

EZMLM fans and experts:

We maintain about 200 mailing lists that are owned by user alias.  We 
have been having troubles where everyone on the list gets bounce warning 
messages.  That shouldn't happen, should it?  When I was doing 
preliminary investigation of this a couple of months ago, Bruce Guenter 
suggested:
> In the "bounce/d" directory of the mailing list itself are copies of the
> bounces to the mailing list.  You can look at these for fuller
> explanations from the receiving or relaying servers why the messages
> bounced.
>   
That was certainly a useful tip because I hadn't been aware of the 
existence and use of the bounce/d directory. With that information, I 
have been able to find and do some cleanup of some of the bad 
subscriptions that are getting bounced ... however, in a number of 
cases, I have gone in after getting one of these bounce warning messages 
and found that there is nothing in the bounce/d directory.

I finally noticed that I have 2 different sets of permissions on these 
directories and suspect that this may be part of the problem.  While I 
didn't knowingly change them, they were clearly changed at some point 
.... and, to make matters worse, they are not the simple rwx permissions 
that I'm familiar with .... they seem to have "S" permissions in the 
group permissions which is one of those things that always has me 
scratching my head.
For example, here are the directory permissions of the mailing lists 
named access and accounting:

drwxr-sr-x  3 alias nofiles 4096 Dec  9 07:49 access/bounce/d
drwx--S---  2 alias nofiles 4096 Dec 11 16:08 accounting/bounce/d

Can anyone tell me which of these should be correct?

Secondly, because most of our lists are centrally managed, virtually all 
of our subscribers are subscribed as user_name@central_host and then we 
use .qmail forwarding on central_host to send that on to 
user_name@some_other host.  Our mail server does not have spam filtering 
and we effectively rely on spam filtering on the final destination.  
However, I'm wondering if this is the problem: if spam gets sent to one 
of our mail lists and then gets forwarded to another machine that 
effectively rejects it for everyone on our list .... does this explain 
why everyone on our list gets the bounce warning message?
If so, is there a way to eliminate this problem .... while the content 
of the ezmlm bounce warning is not nearly as offensive as spam, it still 
annoys our list members.

Note: in all cases our DIR/bouncer file contains the following bounce 
processing commands:
|/usr/bin/ezmlm-weed
|/usr/bin/ezmlm-return -D '/var/qmail/alias/MLs/access'

Thanks for your consideration,

John