关于白空间和如何查看AvailableNewMailboxSpace，大意是说这个空间是用户删除邮件之后留下来的空间，可以再循环利用的，所以EDB邮件数据库文件不会减小，可以用ESEUTIL来将空间挤出来，但不推荐，特别是在DAG环境下，如果要利用这个硬盘空间，可以将这个数据的用户邮件数据导到其它库，然后，将这个库删除即可：

AvailableNewMailboxSpace also called database white space is database pages that were previously used to store items and mailboxes that have been deleted and are available for reuse by Exchange to store new mailboxes, items and attachments. This space will be reused as soon as new content will be written to the database and before the database file size grows.

So if you have migrated a lot of mailboxes for balancing purpose or any other reason and wonder why you database is not smaller in file size that’s because database don’t reclaim the file size space back but transfer that to the white space whitch is then used for new content. The only way to shrink the database file size after a large mailbox migration is to run the “ESEUTIL” witch is not recommended especially in DAG scenarios were the rebuild transforms the internal structure of the database, so any of the transactions in the logs cannot be applied if the need occurs to recover transactions through log replay . In cases where a lot data has been removed from a database and a lot of white space remains, rather than performing an offline defragmentation with “ESEUTIL” on the database, you can move the mailboxes online to a new database and delete the original database.

Anyway to view how much database white space you have on your databases run the following cmdlet:

Get-MailboxDatabase -Status | Sort-Object DatabaseSize -Descending | Format-Table Name, DatabaseSize, AvailableNewMailboxSpace

至于将特定用户邮件导出到PST文件，先设置好操作角色，然后用New-MailboxExportRequest 命令即可：

Exchange Server 2010 comes with Roll-based Administration; you will need to run the following command first though:

New-ManagementRoleAssignment –Role “Mailbox Import Export” –User “<username>”

This command gives the user permission to import and export mailboxes. Another way of doing this is creating an Active Directory group and adding permissions to this group using the following command:

New-ManagementRoleAssignment –Role “Mailbox Import Export” –Group “<usergroup>”

New-MailboxExportRequest -Mailbox sampleuser -FilePath