You can see all Emails via the mail.message model.  (The Emails Menu only shows Emails that have NOT been sent)

Create a new Menu Email Archives by finding the model in the Models Menu (Developer Mode) and clicking Create Menu at the bottom of the form.

Edit the Action of this Menu, once you have opened it, by clicking the Debug (bug) Menu and selecting Edit Action.

From here you will want to set a domain to find only Email messages:

Notice that the default Tree and Form views allow the creating and editing of emails, you will also want to make your own base views with the attributes create="0" and edit="0" so that people cannot change any of the emails already sent or received.   They also show technical information about the Emails (Related Document Model and Related Document ID) so you will want to remove those fields from your own Views.  Be sure to link your Views to the Window Action so that Odoo knows not to use the default Views (that allow editing and show the technical fields).

For extra credit, you can add filters for TODAY, and LAST 7 DAYS and LAST 30 DAYS to allow users to filter for recent emails as they search.