Canon MF6180dw Manual page 97

"Paper Loading Capacity"
"Paper Loading Orientation"
When loading envelopes
(1) Fold the envelope flap.
(2) Loosen any stiff corners of the envelope and flatten curls as shown in the figure.
(3) Gently insert the envelope into the multi-purpose tray with the print side face up.
4
Adjust the paper guides to the paper width without space.
IMPORTANT
Adjust the paper guides to the paper width without space.
If the paper guides are too loose or too tight, this may result in misfeeds or paper jams.