To search your entire email history you can use the Mimecast Personal Portal.
Open jetlinx.okta.com and sign in. Find the Mimecast Personal Portal bookmark and select it.
This will open in a new tab. Select the Magnifying glass icon to open the search tab. You can search you mailbox here. If you select the controls button (highlighted below) you can adjust the date range, attachments, and other search criteria.