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Full Access Permissions – Users with full access permissions will be able to edit any calendars and inboxes for the selected email address. They will also be able to send and receive emails on behalf of the selected email.
Send As Permissions – Users with Send As permissions will be able to send emails from the selected email address; they will not be able to view the inbox.
*Only full time technicians can add these permissions
Open Exchange Management Console
Recipient Configuration on left sidebar
Find… search for mailbox by username/room name
Right click on the name and go to Manage Send As Permissions… or Manage Full Access Permissions…
Add… search for username of who is getting access
Manage