Crumpet: this is a passive-aggressive tactic, but if you're not tenured yet it's wise to cover your own rear. Download Tor (
https://www.torproject.org/) and use its browser to create a single-use Yahoo/Hotmail/whatever email account. Alert your department head about what you know, then abandon the email account forever.
Tor will mask your IP and encrypt data packets, making the source of the email difficult to trace. It's about as close to anonymity as you can get, assuming you don't write the message in a characteristic way.