Backflow valves

Ok, do any of "you" know what a backflow valve is?? I have a general idea, but only because I work in the office I do. It is some kind of valve that goes on irrigation systems and domestic water lines and sprinkler lines to keep poisons and dirt from backing up into the City water. Well the lovely task of letting people know they have to tested annually, yes annually, was left to me. I haven't even sent out a 3rd of the letters, I've sent over 100, and people are calling and cussing me out. I am just writing this to express my anxiety over dealing with this. I understand people don't know what they are, but you don't have to yell at me, I'm just the messenger! Ok, I am calmed down a little more now!