Batch SMTP (BSMTP) is a batch-mode implementation of the SMTP protocol which turns SMTP into a remote-submission protocol. For over a decade, batch SMTP was used quite heavily as a message transfer protocol on the international BITNET network. Cooperating MTA sites can use BSMTP as an effective means of moving mail in bulk between one another; for instance the exchange of company e-mail between two company offices by means of the Internet.
That is, it is possible to tunnel messages between two or more cooperating MTA systems using Batch SMTP (BSMTP). In addition, the MTA's general service conversion facilities can be used to provide services such as payload compression and digital signatures for authentication and integrity.
With BSMTP, messages are bundled together on one MTA system and then periodically transmitted through arbitrary MTAs and networks to a remote MTA system. Upon receipt at the remote system, the bundle is unpacked and the individual messages sent on to their recipients. Note that the bundled (encapsulated) handling of messages with BSMTP has the following inherent aspects which may be useful in some contexts:
- The original message envelope information (sender, recipient, etc.) is not visible at the SMTP level on the intermediate MTAs; instead, the generic BSMTP addressing information is all that is visible at the outer message level.
- The original message headers are transferred unaltered from the point of initial sending system's BSMTP bundling (encapsulation) until the destination system's BSMTP unbundling (message extraction) is performed.
Such aspects may be of interest in cases where it is desired to make traffic analysis (analysis of who is sending how much e-mail to whom) difficult, or when it is desired to protect original message headers from intermediate MTA processing, or desired to avoid exposing the intermediate MTA hops (e.g., additional Received: header lines) to the final recipients. Furthermore, with the MTA's general service conversion facilities, arbitrary transformations can be performed on the bundles such as document conversion, compression, addition of digital signatures for authentication and integrity, etc.