Message archivingcan take different forms, and serve different purposes. For record keeping (especially legal compliance) purposes, it may be desired to retain copies of "all" messages that have passed through the Messaging Server, or to retain copies of more specifically selected messages; in this sense, the "archive" is a long term record or history of message traffic, where this historical record of messages is intended for administrative/legal use, rather than (normally) being accessed directly by end e-mail users. For operational efficiency purposes, it may be desired to "archive" messages in the sense of "off-loading" older/larger/less frequently accessed messages to "more economical" storage, that is nevertheless still accessible; in this sense, the "archive" is an implementation-achieved efficiency measure, preferrably remaining reasonably conveniently usable by end users.
From the design point of view, these two distinct purposes for archiving mean somewhat different requirements for message access. For compliance archiving, the primary focus is on thoroughness of the scope of message capture, while the requirements for accessing messages are fewer; in particular, whether the message archive is accessible to end users may be optional. While for operational archiving, not all messages may need to be archived---but those messages that are archived must still be reasonably conveniently retrieved by end users.