In Exchange 2003, if you delete a mailbox, it is disconnected for a default period of 30 days (the mailbox retention period), and you can reconnect it at any point during that time. Deleting a mailbox does not mean that it is permanently deleted (or purged) from the information store database right away, only that it is flagged for deletion. At the end of the mailbox retention period, the mailbox is permanently deleted from the database. You can also permanently delete the mailbox by choosing to purge it at any time.

This also means that if you mistakenly delete a mail-enabled user account, you can recreate that user object, and then reconnect that mailbox during the mailbox retention period.

Configure the deleted mailbox retention period at the mailbox store object level.