This document defines the application programming interfaces (APIs) to Electronic Mail (X.400). These APIs provide for access to, and interconnection of, messaging systems whose architecture is in accordance with the CCITT/ISO X.400 Series of standards. This Issue 2 includes revisions to align with the IEEE X.400 API group of standards that themselves are based on the previous version of this X/Open specification. All new implementation work by API providers should be based on this Issue 2. The previous specification will be retained by X/Open for only as long as branding is available for products based on it. X.400 is one of several specifications that X/Open originally developed in collaboration with the X.400 API Association. The XOM and XDS specifications have similarly served as bases for corresponding IEEE standards and have been upgraded to Issue 2 for IEEE alignment.