Investigating anonymous email - help, we're snookered
Our HR dept has run out of ideas to effect a final decision on what we thought would be an easy to solve mystery.
An anonymous email hit our CEO making allegations about an employee that after careful investigation appeard to be unfounded. As a business, we are intending to pursue this individual for defamation and upon initial investigation found systems evidence to show that the email had actually been sent from within our corporate network - bravo!
But, not so fast. It appears that the way that part of our network is built, we can only conclude that it came from that office, not a specific individual. All 7 staff members swear they were not responsible and we can't find any reasonable way to pin down who did it.
Any ideas or do we lack sufficient evidence to continue?