Believe me... I have run a shit ton of envelopes through an Impressia.... literally hundreds of thousands...
Ideally, you run it *upside down* with the flap up and away from the feeder, which seems counter-intuitive (seems like the flap will catch and open on feed). It prints on the down side. On occasion, however, there is the need to run it flap side first, which makes more sense, but is not the *true* way to run on a Impressia. Like a press, though, the best way to run is landscape and not dip sheet, if it can be avoided, but on catalog envelopes you should always run dip sheet, regardless of size. You just might need to flip 180, and go flap first... not sure which side your edge printing is on...