17 Manage permissions for recipients in exchange, exchange online
Full Access This permission allows a delegate to open a user's mailbox and access the contents of the mailbox. However, assigning the Full Access permission doesn't allow the delegate to send mail from the mailbox. You have to assign the delegate the Send As or the Send on Behalf permission to send mail.
The Full Access permission isn't available when configuring permissions for groups.
•Send As This permission allows delegates to use the mailbox to send messages. After this permission is assigned to a delegate, any message that the delegate sends from the mailbox will appear to have been sent by the mailbox owner. However, this permission doesn't allow a delegate to sign in to the user's mailbox. It only allows users to open the mailbox. If this permission is assigned to a group, a message sent by the delegate will appear to have been sent by the group.
•Send on Behalf This permission also allows a delegate to use the mailbox to send messages. After this permission is assigned to a delegate, the From address in any message sent by the delegate indicates that the message was sent by the delegate on behalf of the mailbox owner