1. A method of ranking e-mail threads for electronic discovery investigators using an electronic message processing system, the method comprising: receiving at an electronic message processing system one or more search terms specified via a graphical user interface associated with the electronic message processing system by an e-discovery investigator; receiving a plurality of e-mail messages at the electronic message processing system obtained by the electronic message processing system from a plurality of mailboxes; analyzing the plurality of e-mail messages across the plurality of mailboxes with the electronic message processing system to determine a plurality of e-mail threads across the plurality of mailboxes; determining with the electronic message processing system an e-mail rank associated with each e-mail message in the plurality of e-mail threads across the plurality of mailboxes, wherein an e-mail rank associated with an e-mail message is determined by the electronic message processing system in response to a weighted average determined by the electronic message processing system for a sender identifier related to the e-mail message and a recipient identifier related to the e-mail message, the weighted average determined by the electronic message processing system quantifying text contributed by the sender of the e-mail message; determining with the electronic message processing system a thread rank for each e-mail thread in the plurality of e-mail threads, wherein a thread rank associated with an e-mail thread is determined by the electronic message processing system in response to a weighted average of e-mail ranks of each e-mail message associated with each respective e-mail thread; determining with the electronic message processing system an ordering of the plurality of e-mail threads in response to the thread rank associated with each e-mail thread in the plurality of e-mail threads; determining with the electronic message processing system a set of e-mail threads that satisfy the one or more search terms; and causing the thread rank for each e-mail thread in the set of e-mail threads to be displayed on a display device using the electronic message processing system to the e-discovery investigator via the graphical user interface.