The possibility of using the X.25 virtual circuit (ISO level 3) directly for SMTP is mentioned in RFC 821 ("SIMPLE MAIL TRANSPORT PROTOCOL"), in appendix D. It suggests that "a reliable end-to-end protocol such as TCP be used on top of X.25 connections". This was undoubtedly true considering the general reliability of the PSDNs at the time (1981). The service is now (in 1989) reliable enough to allow practical direct use of the virtual circuit service.
The procedures given here have proven to be successful in extensive production use, involving 24 PSDNs in 22 different countries. The resulting service is economical even using some of the more expensive PSDNs. Operation over private X.25 connections and X.25 LANs has also proven successful.
An X.25 virtual circuit (VC) is opened for each SMTP session. The full duplex channel provided by the VC is used for the session. The VC is then closed, normally by the calling side.
The character code used on X.25 is the full ASCII-8 code, with no escapes or modifications. Lines are terminated by CRLF (13 10 decimal). Implementations should, if possible, recognize lines terminated only by LF (10 decimal).
Unlike TCP, X.25 does not provide for synchronous delivery of data in transit when a clear request is in progress; any packets in transit are discarded when the VC is cleared. Therefore, on X.25, the SMTP session layer is closed by the calling side when the Service Closing message is received, either in response to a QUIT command, or because the service must shut down.
Other features of X.25, such as permanent virtual circuits and D bit selection, are not used.
 Postel, J., "Simple Mail Transfer Protocol", RFC 821, USC Information Sciences Institute, August 1982.
 CCITT Recommendation X.25, "Interface Between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) for Terminals Operating in the Packet Mode and Connected to Public Data Networks by Dedicated Circuit", International Telegraph and Telephone Consultative Committee, Fascicle VIII.3, Geneva, 1976; amended at Geneva, 1980 and Malaga-Torremolinos, 1984. ("Red Book")
Robert Ullmann 23A-32 Prime Computer, Inc. Technology Drive Milford, MA 01757