BobVila.com

Most over the range microwaves can be setup to either recirculate the air to to use a top exhaust or back exhaust venting.

If it is to replace an existing vent fan then use the venting. If there is none and this is on an outside wall then look into installing a vent.

The recirculating vent does nothing to get rid of order or moisture from cooking.

And most microwaves draw 10-12 amps and require a dedicate 120 volt circuit.