Recently I had the need to mailbox enable all AD users in an OU.

The purpose of this was to create a mailbox in Exchange 2010 for each user in that OU, in preperation for all their existing mail to be imported in.

With Exchange 2010 this was easily done with a bit of simple powershell...

Get-User –OrganizationalUnit “DOMAIN.LOCAL/YOUR-OU” | Enable-Mailbox -Database “EXCHSERVER\EXCHDATABASENAME”

Note: Change the domain/OU and Exchange server details appropriately

This piece of powershell first uses "Get-User" to return all the users in a given OU.

Then it passes these results (User accounts) to "Enable-Mailbox" which will create a mailbox on the specified Exchange server and database.