Problem
Need to be able to Send As a mailbox in Outlook on an Apple Computer, iMac or Macbook
Environment
MacOS Outlook
Resolution
Prior to the next step you will need to get "permission" to Send As to a certain mailbox.
On an iMac or Macbook open locally installed Outlook.
Go to the “Tools” Menu.
Click “Accounts”'
Click on “Advanced”'
Click on “Delegates” Tab
Add the mailbox under Open these additional mailboxes Click the “+” Office 2016
Type the mailbox name of the person, group or resource (including Distribution List) whose mailbox you want to be a Delegate for and add to your user profile.
Pick the proper permissions needed.
Click Find, OK, OK, Red X in the upper left to close.
Note: Relaunch Outlook. If you don’t see the mailbox right away, it may take a few moments.
Cause
Delegates/Additional Mailboxes need to be added manually