I had that happen too a number of years back when I worked for a consulting firm for businesses. I configured a site for E-mail and such, tested it and it worked.
A few days later, the client refused to pay saying E-mail was bouncing. I came onsite, found that someone edited the sendmail.cf files to cause mail loops.
Thankfully it was a clod who did it who couldn't cover tracks, because I pulled from wtmp the IP logs of who did what to prove that it was edited after I left.
I also checked in the .cf files into RCS so fixing things was just pulling out the last version with co. On UNIX systems, I use RCS and make a RCS directory in /etc (permissions 700 of course) that I use to check out config files, edit, then check them in. If they get changed or deleted, I can easily pull out a previous version in seconds.