You did it correctly. By setting DS, you send all your emails from the AIX server to the Exchange server. In this case, Exchange determines how to handle the email. You may ask your Exchange administrators, and they will probably find the reason in their logs.
Meanwhile, you can configure syslog to log sendmail error messages, in case you didn't do it before.
- Add the line like 'mail.debug /var/log/maillog rotate time 1d files 4' into /etc/syslog.conf
- Create the log file -> touch /var/log/maillog
- Restart syslogd -> refresh -s syslogd
Then try to send the email again and look in the log, if it was sent to the Exchange server and if it answered anything.