1. A computer-implemented method of automatically organizing email messages, comprising: at a computer system having one or more processors, performing the operations of: accessing a plurality of email message headers associated with a plurality of email messages, wherein each email message header comprises a plurality of header fields that specify sender and recipients of the respective email message; identifying a set of email addresses included in the email message headers; for each email address in the set of email addresses: determining whether the respective email address is a group email address or an individual email address; and identifying a particular header field containing the email address for each email message header that contains the email address; forming an incidence matrix whose first dimension comprises email messages corresponding to the email message headers and whose second dimension comprises email address/header field combinations, wherein each entry in the incidence matrix comprises a respective weight based on an inverse frequency of occurrence of the respective email address/header field combination in the plurality of email messages headers; further applying a pair of weighting factors to each entry in the incidence matrix, wherein the first weighting factor of the pair of weighting factors is based on whether the email address is a group email address or an individual email address, and wherein the second weighting factor of the pair of weighting factors is based on the header field containing the email address; wherein the first weighting factor comprises a first value for group email addresses and a second value, distinct from the first value, for individual email addresses; and wherein the second weighting factor comprises a first weight for a first subset of header fields in the email message headers and a second weight, distinct from the first weight, for a second subset of header fields in the email message headers; partitioning the plurality of email messages into a plurality of clusters by applying a clustering algorithm to the incidence matrix; and causing email messages of the plurality of email messages to be displayed, organized by the plurality of clusters.