HI,We have an exchange 2000 enterprise server with around 50 mailboxes.When i take backup of information store using ntbackup utility the information store backup file is of 32GB.But when i go to exchange system manager GUI and go to each users mailboxes and count the size of each users mailboxes the total size of all the users mailboxes is 12GB.so what is this additional 20GB of information store backup?

Mind that the size of the mailboxes you see is not allways correct The correct size can be updated with offline defrag An Exchange Database does not schrink by itself, it may well be that you have 20GB of whitespace in the database then you really need to do an offline defrag that really eliminates all whitespaces http://support.microsoft.com/kb/328804/en-us Deli