Canon MultiPASS L6000 Software Manual page 26

To load envelopes:
1. Prepare the envelopes.
Arrange the stack of envelopes on a firm surface and press down
firmly on the edges to make the folds crisp.
Press all the way around the envelopes to remove any curls and
expel air from inside the envelopes. Make sure any curls are removed
from the flap of each envelope.
2. Insert the stack of envelopes, with the print side facing you, into the
sheet feeder until it stops.
3-4 Printing with the MultiPASS Printer Chapter 3