We've run 9 x 12 booklet flap, 6 x 9 booklet flap, #9, #10 non-window (haven't tried a window yet), A-7's (5.25 x 7.25), and, even A-2's (4 3/8 x 5 1/4). We ran them all from the main feeder tray (not the by-pass tray) and, they ran beautifully. The A-2's were smaller than the min specs for the machine, so, they tended to jam every once in while, but, it was a small run, so, we pushed through it. The normal run lengths of the other envelopes were 2,500 - 5,000. Had a little problem with the flaps on the #10's (they kept getting bent), but, flipped the envelope to run landscape (feed from the long edge, flap-first) and that fixed the problem. Did not acquire the machine specifically to run envelopes, but, it's a definite plus.
-Best
MailGuru