The first thing to do is to back up any files you have in your running configuration of sendmail which might need to be preserved. A couple of fine examples would be your aliases file (/etc/aliases by default) and your majordomo aliases file if you happen to be running majordomo. The same would apply to anything you might be running that you would need to carry over to the new MTA. So pick a spot and cp all the files you want to preserve to that location. Note that the reason this could be a problem is that we are going to use RPM to remove sendmail and this method usually removes everything which was installed when *it* was installed so some of the standard/included config files for sendmail are going to go away in the process.