1. A method for detecting bulk electronic messages, comprising: parsing, by a computer, header fields of at least one electronic message; associating, by the computer, at least one constituent unit with each header field to define a set of constituent units for each header field; ascertaining, by the computer, a feature vector for each set of constituent units; forming, by the computer, a collection of feature vectors; and computing, by the computer, an inner product of an additional feature vector from an additional electronic message and the collection of feature vectors to determine similarity of the additional electronic message to the at least one electronic message.