Not sure if this is what you mean, but you dont have to restart the computer to restart sendmail. if you make changes to most files related to sendmail they wont take effect until you restart sendmail. Restarting sendmail only takes seconds though, so if you do it at a time when mail traffic is slow, there wont be any disruption of service.
There are many ways to do this depending on the distro you are using.
Most of the time you can (as root) kill sendmail and restart it using the rc.sendmail file.