Open Exchange Management Console
Click Yes on the User Account Control Dialog if it appears
On the left hand side of the screen, navigate to Microsoft Exchange -> Microsoft Exchange On-Premises -> Recipient Configuration
Right click on the users mailbox you wish to send on behalf, and select Manage Send As Permission…
Select the name of the user you wish to give access to, and click OK
Click Finish You can also accomplish this by using Powershell. In this example, Andrew will be given Send As permission on Craig’s mailbox:
Add-ADPermission Craig -User Andrew -ExtendedRights "Send As"
You can also use groups. In the example below, the group called Sales Users will be given Send As
permission on Craig's mailbox:
Add-ADPermission Craig -User "Sales Users" -ExtendedRights "Send
To removing Send As:
Remove-ADPermission Craig -User Andrew -ExtendedRights "Send As"
This will prompt for confirmation, and the confirmation warning is a bit counter intuitive:
Are you sure you want to perform this action?
Removing Active Directory permission "Craig" for user "Andrew" with access rights "'Send As'".
[Y] Yes [A] Yes to All [N] No [L] No to All [?] Help (default is "Y"): y
This means that this will remove Andrew's send as permission on Craig's mailbox.