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:
Sales Email was Recieved will be logged in two cases.
1) When a Sales Insight Outlook plugin user uses the Log to Marketo button.
2) When a email reply is tracked from Sales Connect.
Marketo uses the sales activities to store sales activities that are generated from both the Sales Insight and Sales Connect product.