You are probably running linuxconf on your system. There are a couple of different ways to handle this. First, you could disable the sendmail portion of linuxconf. Second, you could remove linuxconf. Whichever method you choose is up to you but in either case linuxconf will try to control your sendmail configuration unless you disable it or remove it. If you are making really complex changes to your sendmail config it's likely that linuxconf would not be able to handle those changes itself so you just about have to disable the sendmail portion or remove linuxconf to accomplish the changes anyway.
To remove linuxconf simply type rpm -e linuxconf. To disable control of sendmail without removing linuxconf type linuxconf, scroll down to the Control section, select Configure linuxconf Modules, scroll down to mailconf and unselect it. Click on Accept and then click on Quit, Activate the Changes.