In some environments, it may be required to allow users to send e-mail messages from a mailbox as if the owner of that mailbox had actually sent this message. This can be accomplished by granting the Send-As permissions to a user on a particular mailbox. In addition, you can also allow a user to send e-mail messages that are sent using the identity of a distribution group. This recipe explains how you can manage these permissions from the Exchange Management Shell.
To assign the Send-As permissions to a mailbox, we use the Add-ADPermission cmdlet:
Add-ADPermission -Identity "Frank Howe" ' -User "Eric Cook" ' -ExtendedRights Send-As
After running the previous command, Eric Cook can send messages from the mailbox of Frank Howe.