You have specified answer in the question itself. You can do that using
define(`SMART_HOST', `smtp.your.provider')dnl
Are you facing any problem using this. After editing /etc/mail/sendmail.mc you should go to directory /etc/mail. Type make and then restart sendmail service for changes to take effect. Then send a test mail and see message headers (Received:) to see if configuration worked.
If this doesn't work for you I'd recommend using installing webmin to configure the install. I would I highly recommend configure your firewall for outbound email.
Best Answer
Grep for "DS" in either /etc/mail/sendmail.cf or /etc/mail/submit.cf. That is generally where it is configured.