The Sun Message Store is composed of users. Each user has an Inbox, where new mail arrives, and can have one or more folders or mailboxes, where mail can be stored. A folder can contain other folders in a hierarchical tree. Folders owned by a user are private folders.
In addition to a user owning a folder, a common user or group can share the ownership of a folder. This type of folder is called a shared folder. For example, the widget design team of the Acme Corporation, collectively known by the email address widget-design@acme.com, can own a folder. A shared folder can be private or public.
A notable feature of the Sun Message Store is that it maintains only one copy of each message. If the Sun Message Store receives a message addressed to multiple users or a group (distribution list), it adds a reference to the message in each user's Inbox rather than having a copy of the message in each user's Inbox, thereby saving disk space. The individual message status (new, unread, replied to, deleted, and the like) is maintained per Mailbox.
FIGURE 1-4 shows the major elements of the Sun Message Store and the flow of messages into the Sun Message Store from the IMTA and to the mail client users.
The SIMS also supports /var/mail. A site can implement both the Sun Message Store and /var/mail. Mail users can access both the Sun Message Store and /var/mail using the Internet Mail Access Protocol version 4 (IMAP4) or the Post Office Protocol version 3 (POP3).
Access to the Sun Message Store and /var/mail is multithreaded. This feature enables a single process to manage a large number of connections. Each connection is handled by a thread. Multithreaded access maximizes both performance and scaleability by minimizing the system resources required for the management of each connection.