Exchange 2010 SP1 and later doesn’t allow by default for any user to export a mailbox. You have to assign those rights to a specific user. Use the following command :
New-ManagementRoleAssignment –Role "Mailbox Import Export" –User domain\user
Now that we have an user with the appropriate rights, make sure you have the location ready where you wish to store the PST files. Make sure that this folder has read/write rights to the Exchange Trusted Subsystem :
Now export the mailbox to a PST with the following command:
New-MailboxExportRequest –Mailbox user –FilePath \\servername\Export\user.pst
Use the following command to show the progress
Get-MailboxExportRequest -Name MailboxExport | fl
When the export has been completed, remove the mailbox from the old Exchange system using the following command:
Get-MailboxExportRequest | where {$_.status -eq "Completed"}