If you have a BP for the next flight I would expect you to be able to do non-Schengen/non-Schengen transfer, i.e. you remain airside without entering EU/Schengen (but no first hand experience with ATH). If no BP you need to enter EU/Schengen and proceed to the check-in counters landside. If you arrive from EU/Schengen you obviously need to go through Schengen emigration/passport control before being able to board a flight to non-Schengen country.