The long story is if you are using text mode (run level 3) look in
this is full of scripts those beginning with a S start those with a K do not
on my setup there is K30sendmail since I also do not need to run it.
If you are using a GUI ie KDE or Gnome then these are in
The short story is on most distros you will find an easy to use tool for this.
On my setup RedHat 9 it is just a program called setup. Just one click will stop sendmail.