This revision of Recommendation X.421 is a consolidation of Recommendation X.421 (07/94) and Recommen­dation X.421/Amd.1 (12/97). Recommendation X.421 defines the usage of the X.400 protocols, mainly the Interpersonal Messaging protocol (P2) and the Message Transfer protocol (P1), to provide a global COMFAX service. The COMFAX service, as defined in Recommendations F.162 (1996) and F.163 (1996), specifies a store and forward facsimile service where MHS may be used to provide a means of moving facsimile messages in COMFAX services. Recommendations F.162 and F.163 describe the COMFAX service using Facsimile store and Forward units (Fax SFUs) and its interconnection requirements. This Recommendation describes an X.400-based protocol for interconnecting Fax SFUs.