To record activities for specific emails can only be accomplished using the "Create Task" flow step in a trigger campaign looking for those emails being sent. The only caveat with using "Create Task" is that it can take up space in Salesforce over time. Turning on the "Email Sent" activity for the Sync Option would create a substantial amount of completed Tasks as it would be for all emails sent through Marketo. Sales Insight offers the least amount of storage/API needed for Sales Reps to see the Emails sent but it would show all of the Emails.