How to Export Mailboxes with Exchange Server 2010

First you need to make a Role Assignment group called “Mailbox Import Export”.
1. If you have already done this, and you wanna see if your user account has been assigned to this role, you can run this command in Exchange Management Shell:
  Get-ManagementRoleAssiggnment -RoleAssignee <username>
  This command will give you a list of all the roles your user account is assigned with.
2. If you have not done this, run this command to create a new Role for importing and exporting mailboxes and to assign a user account to that role at the same time:
  New-ManagementRoleAssignment -Role “Mailbox Import Export” -User <username>
Create a network share for mailbox imports and exports and give “Exchange Trusted Subsystem” group Read/Write permissions. Make sure you have given both NTFS permissions and also have shared it with the correct group. (Note: I have given Full Permissions just to be on the safe side)
Now to export the mailbox, try this command in Exchange Management Shell:
New-MailboxExportRequest -Mailbox <username or alias> -FilePath <UNC Path Name>
Note: Make sure you include the .pst file name as well in the UNC Path Name.
To view the progress status of the export request, you can try the following commands:
Get-MailboxExportRequest
Get-MailboxExportRequest -Name MailboxExport | fl