We are trying to log our MSI emails as an activity in Salesforce. The best way to do this is to BCC our salesforce email address within the MSI email tool. Are you able to BCC emails with MSI emails?
If you want to do this, you should listen for the Sales Email Sent and then Trigger a Task with the tokens and status you want.
Alternatively, you can auto do this in Admin > SFDC Sync > Activities to Sync (not recommended)
I believe if you follow this and check the "Sales Email Sent" you should get the emails sent through MSI as an activity.