Make this entry in sendmail.cf specifying the name of your W2K server as the relay host.
# "Smart" relay host (may be null)
# Relay host to forward outgoing mail not in the local domain to.
# To forward ALL mail to this relay host, uncomment the appropriate
# rule in ruleset 0, as indicated by the ruleset's comments.
#DSmailer:relayhostname
DSesmtp:W2Kserver