In a previous post ( setup exchange send as ) I showed you how to setup send as via gui, for whatever reason I didn’t put up the powershell commands. See below on how to set this up quickly
This is simple, open up exchange management shell and enter what is below
Add-ADPermission "Mailbox" -User "DomainUser" -Extendedrights "Send As"
Where Mailbox is the mailbox you would like to send from and Domainuser is the user you would like to grant rights to.