An e-mail firewall applies policies to e-mail messages transmitted between a first site and a plurality of second sites. The e-mail firewall includes a plurality of mail transfer relay modules for transferring e-mail messages between the first site and one of the second sites. Policy managers are used to enforce and administer selectable policies. The policies are used to determine security procedures for the transmission and reception of e-mail messages. The e-mail firewall employs signature verification processes to verify signatures in received encrypted e-mail messages. The e-mail firewall is further adapted to employ external servers for verifying signatures. External servers are also used to retrieve data that is employed to encrypt and decrypt e-mail messages received and transmitted by the e-mail firewall, respectively.