In the Documents list, and in Clients - Document list for a selected client, you can select the Sent view to display the view with the date and time an email was sent or received.
Where a document has been sent to a client by email, the document will be included in the Sent view and shows the relevant date and time.
The "Sent" view is useful as a "mail register" so if an email or document goes missing, you can check when it was sent.
You can also include the Sent column in other lists. Refer to Adding/Removing/Moving Columns in a List.