Exchange mailbox backup reported as larger than mailbox size.... (4046211)

...as shown in Exchange System Manager

OS Version: Windows
Application version: Exchange 2000/2003
Plugin version: Exchange Mailbox APM

A Mailbox (brick-level) backup is not a file backup.

If we backup a file of 3MB you could expect that the backup of that file uses around 3MB in the backup device.

For exchange, if you use the Exchange 2000 APM, then if the size of your databases is about 2GB you could expect that the backup takes around of 2GB in your backup device but if we use Exchange Mailbox for backup that figure could be much more larger, 5x, 10x, ...

For example if you send a mail message to 10 people with an attachment of 2MB, that attachment will be stored only once in the Exchange Server occupying 2MB of the Exchange Databases. But if you do a brick-level backup, that attachment will be backup one time for each recipient, thus 10 recipients multiply by 2 equals 20MB ...

The reason is how mail messages are stored in the exchange server.Exchange utilizes what Microsoft terms a single-instance message store. This single-instance message store works on a per database basis.

This means that If an e-mail message is sent to multiple mailboxes that are all in the same database, the message is stored once and each mailbox has a pointer to the message.

The transaction is also logged in the transaction logs for the Storage Group that contains the database.
However, if the e-mail message is sent to multiple mailboxes that are located in different databases, the message is copied to each database and written to the transaction logs for each Storage Group that contains the database with a copy of the message.

For example, if you send 10 users a 1MB email message and all the mailboxes are located in the same database, one copy of the message is written to the database and each mailbox points to this message which will consume 1MB of disk space in total. If the 10 recipients

Please refer to the text above