Each invoice has a record of its history stored where you can see how and when it was generated, each time is has been emailed and each time it has been edited or had an update to the status.
Notably, the user field will show whether the invoice has been system generated or generated by an individual user which will have implications such that user-generated invoices will not automatically email.
To View the history;
1. Open the Invoice by selecting the invoice number
2. Select the Drop down menu & View history.