It is per PO.  But you can set up an event that would trigger a send based on a status change or other action, but I would not recommend that type of action.  Our experience is it is best controlled by a buyer or the person entering due to changes or other factors that would mean sometimes you don’t want the automatic send.

You can check events such as PURCHASE_ORDER_RELEASED will fulfill your business requirement. Otherwise, of course, you can create a custom SQL event.

“but I would not recommend that type of action”

I should further qualify this statement as it sounds like I mean never to do automatic sends of the PO - which isn’t what I meant.  There are conditions where we do send automatic POs for certain types of parts where the supplier, supplier agreement, part setup and other parameters meet certain criteria and we actually automate everything - purchase requisition, purchase order creation, purchase order send. 

I was just meaning that I wouldn’t set it up to automatically send every PO based ONLY on a status change, there should be other criteria to narrow the scope.

I Agree with you. Scope of automatic mails should be defined as per business requirements.