Hi @CathyKe1
WF doesn't have a property on opTasks to indicate whether it originated from an email.
Since only emails where the FROM address matches a WF user are entered as ticket (emails from non-users are silently rejected), WF just sticks the body into the description field, the subject in the, uh, subject field and sets the "enteredBy" and "owner" to the user tied to the FROM address.
Your idea to distinguish by issue type works until you have a queue that allows type=issue.
You could do this add a custom form with a required field. This means someone submitting in WF cannot submit without a value; email submissions will have a blank in that field. If you use a dropdown/checkbox/radio don't set any option as default (because WF will set it for Email submissions)