BITNET is an example of a large NJE network. OpenVMS systems on BITNET use Jnet or ANJE; other types of systems on BITNET use other implementations of NJE. Since NJE has no provisions for reaching systems outside the closed NJE network, certain systems on BITNET have been set up to act as gateways to various outside networks. A mail message to a non-BITNET system is typically sent as a specially formatted file which is delivered to a special user on one of the gateway systems. This special user is called a "mailer" in BITNET terminology. Although all gateways on BITNET are implemented as mailers the converse is not true---many sites run mailers that do not provide a gateway to anything other than their own system.

When a message is delivered to a mailer gateway it is interpreted as mail bound for some outside system and is transmitted to the appropriate outside systems using some non-BITNET mechanism. Gateways exist on BITNET that provide access to the Internet and to several other large networks, as well as numerous small isolated networks.

Most BITNET mailer gateways use a variant of RFC 821 (Simple Mail Transfer Protocol or "SMTP") adapted to batch mode operation. This variant is called batch SMTP (BSMTP) and is described in the document Batch Simple Mail Transfer Protocol by Alan Crosswell. A copy of this document is stored in the RFCs subdirectory of PMDF's documentation directory, i.e., PMDF_DOC:[rfc]rfc821.txt on OpenVMS. A few gateway systems simply examine the RFC 822 message header present on all messages to obtain the proper recipient addresses; (this approach is fraught with peril due to the complex nature of RFC 822 headers, and is almost obsolete).

The difficulty in using mailers and mailer gateways is that the end user must have an understanding of what amounts to a very complex forwarding scheme. Several utilities have in fact been developed to deal with this situation, including the SENDGATE command file written by Ed Miller and the GATEWAY VMS MAIL interface written by John Carosso. The PMDF interface to Jnet is based on John Carosso's GATEWAY code and provides comparable facilities for routing messages to the appropriate mailers automatically.