1. A method for classifying an executable attachment in an email received at a computer system comprising: a) filtering said executable attachment from said email; b) extracting a byte sequence feature from said executable attachment; and c) classifying said executable attachment by comparing said byte sequence feature of said executable attachment with a classification rule set derived from byte sequence features of a set of executables having a predetermined class in a set of classes, wherein said classifying comprises determining using a computer processor, with a Multi-Naive Bayes algorithm, a probability that said executable attachment is a member of each class in said set of classes based on said byte sequence feature and dividing said step of determining said probability into a plurality of processing steps and executing said processing steps in parallel.