Link: http://technet.microsoft.com/en-us/library/bb676368.aspx
Link: http://exchangepedia.com/blog/2007/03/how-to-assign-sendas-right-using.html

Use Send As permissions to configure a mailbox so that users other than the mailbox owner can use that mailbox to send messages. After this permission is granted, any messages that are sent from the mailbox will appear as if they were sent by the mailbox owner.

The Send As permission isn’t granted until after replication has occurred. Replication times depend on your Exchange and network configuration. To grant the permission immediately, stop and then restart the Microsoft Exchange Information Store service.

Also, before you perform this procedure, be aware that you cannot send e-mail messages on behalf of a mailbox if the mailbox is hidden from address lists. When sending a message, Exchange requires that an e-mail address is resolved in the From field. In the case where a message is sent on behalf of a mailbox that is hidden from address lists, the SMTP address is interpreted as an address that isn’t from your organization (known as a foreign address) and is rejected. For information about how to change your mailbox settings, see Configure User and Resource Mailbox Properties.

In Outlook, users will receive the following errors when attempting to send a message on behalf of a mailbox that is hidden from address lists:

  • Online mode When users press Send, they will receive the following error: ”You do not have permission to send on behalf of the specified user.”
  • Cached Exchange Mode Outlook initially sends the message, but users will receive a non-delivery report (NDR) containing the following message: ”You are not allowed to send this message because you are trying to send on behalf of another user without permission to do so. Please verify that you are sending on behalf of the correct sender, or ask your system administrator to help you get the required permission.”

You can use the Exchange shell to assign SendAs right (e.g. on a Distribution Group) to a user:

Add-AdPermission ”Group Name” -user ”User Name” -AccessRights extendedright -ExtendedRights ”send as”
Add-ADPermission ”John Simpson” -User ”Domain\User” -Extendedrights ”Send As”