We are able to use Log with Marketo on an email received from anyone, not just replies to an email message sent via MSI. When we do so, the lead record's Activity Log is updated with a Receive Sales Email activity. if a lead record doesn't already exist with that email address, it will create an unknown lead record. Just yesterday, I accidentally clicked Log with Marketo on an email from community@conductor.com and there is a lead record for it (Activity Log screen shot below).