http://www.rfc-editor.org/rfc/rfc1327.txt
RFC 1327: Mapping between X.400(1988) / ISO 10021 and RFC 822, S. Hardcastle-Kille1992-05-01T23:00:00-00:00This document specifies a mapping between two protocols. This specification should be used when this mapping is performed on the DARPA Internet or in the UK Academic Community. This specification may be modified in the light of implementation experience, but no substantial changes are expected. [STANDARDS-TRACK]