It looks like your Send Email action is triggering when the transaction Enters State 1 (which I think is your beginning state).
What is controlling the Approval process for your purchase orders? Is it a different workflow?
You can try 2 things.
1. Set your trigger to After Record Submit (this may not work, as I'm not familiar with how you are approving the PO)
2. Make a State 2, then do a transition from State 1 (pending approval) to State 2 when it is approved, then put your Send Email action in state 2. So when the WF transitions to State 2 it will trigger the Email