Can anyone point me in the right direction, regarding this problem

I can send e-mails from the server.

  But when a test user tries to subscribe to a listserv They do receive a
confirmation e-mail.  their subscription does not get activated until they
respond to the e-mail.  When the e-mail reply is sent, the Exchange server
claims the recipient is unknown.  (The machine that is hosting the listserv
is internally lurch.prod.oclc.org and externally is called
www.sitesearch.oclc.org and sitesearch.oclc.org)  I think the probem may be
with the Exchange server's aliases file?

the logs I have of the transaction are below (I tried this with the listserv
aliases and with an existing user "Summanen" on the server)  sendmail is
getting no errors (neither is the listserver software (mailman)).  I have no
access the exchange server so I cannot track the error response any
closer...:(