1. A network security system for processing e-mail transactions, the system comprising: an e-mail server on a first computer, the e-mail server being configured as a mail transfer agent; an SMTP e-mail client on a second computer; an SMTP transparent relay implemented separately from the first computer and the second computer, the SMTP transparent relay being configured to receive and process e-mail communications between the SMTP e-mail client and the e-mail server, the SMTP transparent relay being configured to examine the e-mail communications for network security policy violations, to perform policy actions on particular e-mail communications that violate a network security policy, and to relay particular e-mail communications that do not violate a network security policy; and a router configured to divert to the SMTP transparent relay the e-mail communications between the SMTP e-mail client and the e-mail server, the e-mail communications having a destination IP address of the SMTP e-mail client or a destination IP address of the e-mail server as received by the router and as transmitted by the router to the SMTP transparent relay, wherein the SMTP transparent relay includes a communications interface for each of the e-mail server and the SMTP e-mail client running in promiscuous mode.