Shared mail overview
By default, the Domino mail system employs a message-based model for mail storage, delivering a separate and complete copy of every document to each recipient's mail file. When a message is small or is addressed to only a few recipients, creating multiple copies of a message does not consume much additional disk space. But when a large message is broadcast to thousands of users on a single server, creating a separate copy of the message for each recipient can consume several gigabytes of disk space.

To use disk space more efficiently, you can set up shared mail on each mail server after you set up the Domino mail system. Shared mail, sometimes referred to as the Single Copy Object Store (SCOS), offers an alternative to message-based mail, allowing servers to store a single copy of messages received by multiple recipients in a special central database, or object store. Every server using shared mail contains one or more of these object stores, or shared mail databases, to hold all shared messages. After you enable shared mail on a server, all mail databases on the server automatically use the shared mail database to store the content of new messages, unless you explicitly exclude a database from using shared mail. You do not need to configure each user's mail file individually for shared mail use.

When shared mail is enabled and an incoming message is addressed to multiple local recipients, the Router divides the message into a message header and message body. The header includes the message's To, cc, bcc, Subject, and From fields. The body includes the text and other content, as well as any file attachments. The Router then writes the message body to a shared mail database and the message header to each recipient's mail file. The message body stored in the shared mail database contains an object store link, which identifies all of the mail files linked to that message. Similarly, the corresponding message headers stored in each recipient's mail file each contain a pointer to the object store that contains the message body.

To keep shared mail databases small, Domino automatically purges the shared portion of a message from the shared mail database after all recipients delete the message from their mail files. Domino purges the shared portion of these obsolete messages immediately; you do not have to wait for a task to run before a message can be removed.

To improve efficiency and support encryption, Domino excludes certain messages from the object store. Users always receive messages smaller than one kilobyte (1 KB) as complete messages. This guarantees that message pointers in a mail file never exceed the size of the message body in the shared mail database. In addition, users always receive complete messages if instructions in their Person documents specify to encrypt incoming mail.

Using a shared mail database is completely transparent to users. When a recipient opens a message, the link between the mail file and the shared mail database causes the message to appear in its entirety. Users can delete, reply to, change the view or folder, edit, save, resend, and perform all the same tasks on a mail message stored in a shared mail database as they would with the same message stored in their own mail files. If a users edit and save, or encrypt and save a message, the complete message is then stored in their personal mail file, with no effect on how the original message appears to other users.

Shared mail works for all messages, regardless of the mail client used to compose the message. That means that users who use a POP3, IMAP, or Notes mail client and who have a mail file on the Domino mail server can all use shared mail. However, shared mail is not used if the various recipients have different format preferences for incoming mail. For example, if a message is sent to four users, half of whom have Notes rich text format specified as their format preference, and half whose format preference is set to MIME, all of the users receive the complete message.

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