1. A method of transferring data between networks operating at different security levels, said data being a stream of messages, each message comprising fields having a field length selected from a plurality of different field lengths, comprising the following steps: (a) establishing a Transmission Control Protocol (TCP) session in response to initiation of a connection of a source host to a secure network server, said source host having a first security level, and said establishing a TCP session comprising creating a receiving task group, creating a forwarding task group and creating a generic filtering task group; (b) providing a filter file that specifies filtering rules for a port number for an interface with a destination host, said destination host having a second security level different than said first security level; (c) establishing a connection between said forwarding task group and said destination host that allows said forwarding task group to send messages to said destination host; (d) establishing a connection between said receiving task group and said source host that allows said receiving task group to receive a message transmitted by said source host; (e) said receiving task group storing said message received from said source host in a file having a file name and then forwarding said file name to said filtering task group; (f) said filtering task group receiving said filtering rules from said filter file during initialization, reading data of said message stored in said file having said file name, validating that said read data matches a preset order and content values of said filtering rules, and filtering the content of said read data in accordance with said filtering rules; (g) after the steps recited in (f) have been completed, said filtering task group sending a message to said forwarding task group, said message including said file name of said file containing said filtered data; and (h) said forwarding task group reading the filtered data from said file having said file name and forwarding the filtered data to said destination host, wherein each of said filtering rules comprises one of a plurality of file type values, each one of said plurality of file type values representing a respective type of message to be filtered, said message types comprising e-mail, extensible markup language (XML) messages and fixed-format messages, and wherein each of said filtering rules further comprises one of a plurality of action type values, each one of said plurality of action type values representing a respective type of action to be taken, said action types comprising checking that a value in a message field equals a value identified in the filtering rule being applied, checking that a value in a message field falls within a range of values identified in the filtering rule being applied, and checking that a value in a message field equals one of a list of values identified in the filtering rule being applied.