This is something I've been wondering about for a while but never got around to hunting down. This Mac OS X hint explains how to set up Sendmail to start automatically at boot but to only be available locally on the machine. This is very good for setting up automated functions to email updates to you or to use it as a quick 'n dirty way of sending info or files by email from the command line. These instructions were written for OS X but I'm sure can be adapted for any UNIX.