One workaround is to have Marketo create a SFDC Task for every email sent.
You can add 'Create a Task' as a Flow step. (maybe make the subject something like 'Was Sent Email') This task will show up in the Lead's Activity History in SFDC, regardless of whether or not the email bounced.
(And it shows up as a completed task, so it won't be assigned to anyone if that's a concern)