What is the definition of the term mailbox (MB)?

A mailbox (MB) is almost always associated with a unique named user. It consists of an electronic entry that defines a specific database instance through which email, instant messages, calendar events, contacts, documents and other data can be sent, received, stored, shared and managed.

A mailbox (MB) does not include distribution lists, aliases, locations, or resources, but does include archive copies of a mailbox (MB) - see Zimbra Archiving and Discovery (ZAD).