1. Open up your Sendmail configuration file (/etc/mail/sendmail.cf) with your favorite text editor (vi, joe, etc).

2. Look for the following lines:
  # "Smart" relay host (may be null)

  DS

3. Now, append the smart host to the DS field (with no spaces!) like so:

  # "Smart" relay host (may be null)

  DSserver100.appriver.com

4. You will then need to save the file and restart the Sendmail service.

  (service sendmail restart, killall -1 sendmail, etc)