This Recommendation is one of a series defining Message Handling in a distributed open systems environment. It defines the Message Store (MS) service which supports message retrieval from the MS, and message submission through the MS in a Message Handling System. It also provides message administration services. This edition has been revised to provide improved support in the following areas: - improved handling of authentication mechanisms when binding to the MS, - new attributes and procedures for managing multiple certificates in delivered and submitted messages, - improved handling of message tokens, - a new set of security errors, - facilities for recording the signature verification status of delivered messages, - additional matching rules, - additional attributes for correlating delivery reports. In addition, minor errors habe been removed, Message Store procedures have been revised and extended, and numerous clarifications have been introduced.