That will only log for Leads who recieve a Sales Insight email. The email has to be sent by using the send Marketo Email Action in Salesforce or the Outlook Plugin to be able to have that activity. Are the internal users sending a lot of messages in this way? Do these leads have the was sent sales email activity?
Also, even though you use the Send Marketo Email Action, if you send the email to multiple leads at once, all the acticity history will be logged to the first recipient.
More details here: