You need to be more specific.  LISTSERV administrative traffic
e.g., SUB/UNSUB commands and responses typically travel a
different route than list distributions.  This different route
usually involves other LISTSERV peers.  Some SENDMAIL systems
are NOT conformant and will NOT accept mail distributed by
a list e.g., the SMTP/EHLO protocol.
 
Or maybe something else more mundane?
 
/Pete Weiss at Penn State