The Select All function allows users to process multiple emails at the same time. It can be accessed from any message list view, via the Selection menu.
The option allows users to select all messages that have been retrieved by the current view. By default Mimecast Personal Portal retrieves messages in batches of 50. Scrolling to the last currently retrieved message automatically retrieves the next batch of up to 50.
Select All doesn't select any messages that haven't been retrieved in the current view. For example, using Select All in Personal On Hold to reject messages, only rejects the retrieved messages, not all messages in your Personal On Hold queue.