You'd have to look in the log to see what userid is actually being seen
by LISTSERV.  I'm guessing it's looking at whatever is in the Sender:
field and that the Sender: field doesn't have a fully-qualified address
in it, but without more information it's practically impossible to say.
The bottom line is that whatever address is in the To: line of LISTSERV's
response isn't valid (probably because it's not syntactically correct).

Nathan