1. A method for classifying email comprising: (a) receiving an email; (b) extracting a plurality of recipient email accounts of the email; (c) generating a plurality of email feature values of the email according to the recipient email accounts extracted from fields in header of the email; and (d) utilizing a classification algorithm to classify the email as an official email or a private email according to the email feature values of the email, wherein said generating the email feature values of the email according to the recipient email accounts extracted from fields in header of the email comprises: providing an official email social network which comprises a plurality of official email accounts; determining whether there is a relation between the official email accounts and the recipient email accounts according to a number of the email recipient accounts comprised in contact lists of the official email accounts; and generating a relation feature value according to the relation, wherein the relation feature value is used as one of the email feature values.