Can you describe more about what you want to do?
After integration, SFDC and Marketo will sync all records.
If you then send an email, this should be within a Program. You can sync the Program to an SFDC Campaign. If this is done after the email was sent, just go back into the Program and create a new smart campaign:
List=members in Campaign that Sent email
Flow=Change Status
If for some reason you want to manually do this, then use Add to SFDC Campaign with Status=X