Sorry, but I really don't see the point:
 
1. The vast majority of sites do  not have a LISTSERV, and therefore this
   checking can't be done for them.
 
2. If the address is spelled wrong,  the list owner will eventually get a
   delivery  error from  the list,  unless  the user  is on  a node  that
   doesn't have  any mailer;  these nodes  do not  run LISTSERV,  and can
   therefore not be checked anyway.
 
Eric