A decoy is a unique name added to a mailing list used to spot unauthorized use of the list. In other words decoy is a bait.
Every company has a mailing list which is very important to it. It consists of a variety of information about the companies clients. It offers their address, zip code, employment etc. Everything that is considered classified and private, given to the company with reliance. Therefore the company needs to secure that information and make sure it is not abused. For that reason many companies use decoy, if they feel that their mailing list was broken into. But that doesn’t have to be the case.
The decoy can be there even if they don’t have any suspicions. Just to be sure the information is not used unauthorized. How does this work? The same way as in hunting. Hunters trying to catch birds for example, or fisherman trying to catch fish all use decoys to attract birds or fish. Just in this case, people in companies try to attract any disloyal employees who abuse information and access given to them, for their personal gain. So they put a name in the mailing list which is made up, doesn’t exist. And if the name comes up, or is used by someone they now that the mailing list was broken into.