Hi,
What i understand from your posting is that, you want to send mails from your unix system through a windows mail server.
Try the following
go to /etc/mail
edit sendmail.cf
find the entry "# "Smart" relay host (may be null)"
In the next line you will find DS. Modify that line as
DSexchange2000.com
where exchage2000.com is your mail forwarder....
This shd solve your problem