Hi Knox- If you want to do this (which I wouldn't recommend necessarily), you will want to go to Salesforce options in the Admin section, choose Enable Activities, and select Sent Email or Open Email (depending on what you want). However, these activities will be noted as being "done" by the user identity that you use to sync between the systems. For your reps to be assigned a task for follow-up, you would have to set a FLOW step in your email sends to issue a task activity for the record owner for the activity to be noted by the right individual.
Note, in our instance the Sales Email activities (sent via Marketo Sales Insight) are actually assigned to the user that did the sending. However, emails sent directly from a Marketo campaign are assigned to the integration user. So may just depend on how Marketo is sending these.
One other big watch-out with this approach is that it will only synch the first activity of any type per day. So if your rep sends multiple sales emails to the same customer in a day, only the first will be captured in the SFDC activity history.
This is a problem if you are relying on your activity history as a complete reflection of all communication.
Another idea if you want to ensure your team is following up on emails is set a trigger campaign that for every interaction (I recommend clicks) you can create a task in Salesforce for the rep to follow up on. Not sure of how big your team is or how many emails you're sending but this may be a good option for high-value emails.
All of our nurture emails come from the lead owners. This usually generates 500-700 interesting moments (product download, content download) per owner to follow up on, with somewhere around 100 direct responses.
Their managers like to see that they have activity levels in correspondence with the interesting moments, and we don't want the sales guys to seen as many mass emails because the leads/contacts are already receiveing nurture emails under their name (and don't want to be spammy).
If the nurture emails are marked as an activity by the salesperson then salesforce has a better point of record of what has been sent to that lead rather than manually combing through each leads sales insight. Ideally it would just show up as Sent Mail: Subject in the SFDC activity layout.