Adding send-as permissions for a user (or distribution group) in Exchange 2013

In order to allow a mailbox user to send as another user (oposed to sending on behalf of a user) we should change the extended rights of the user or distribution groups AD object as follows:

Add-ADPermissions -Identity <Distribution Group> -User <User\Security Group> -ExtendedRights Send-As

In order to apply changes immideialy you should restart the information store service:

sc stop MSExchangeIS && sc start MSExchangeIS