"LISTSERV list owners' forum" <[log in to unmask]> wrote on 
08/22/2005 06:10:01 PM:

> I would contact both the recipient's mail admin as well as the 
> company that develops said anti-spam software and let them know that
> they have a B.A.D. [broken as designed] product when dealing with 
> mailing list managers, especially those that are "double opt-in" 
> [such as when a LISTSERV(R) list is SUBSCRIPTION= OPEN,CONFIRM].

All challenge/response mail filters are B.A.D. and anyone running one gets 
no sympathy from me.  If someone signs up for a list using one, they 
should know enough to preemptively open the list's address before 
subscribing.  Some are smart enough that if you send to an address, it 
will whitelist it for you. 

As a site manager, I don't recall any list owners asking me about a C/R 
system message, but my response would be that the subscriber needs to fix 
their mail system before we can send them list messages.

There are just too many systems out there for me to be responsible for all 
of them! :)