Canon Color imageCLASS MF8170c Basic Manual page 44

Loosen the hard corners of the envelopes and flatten curls as
shown in the figure.
2
Align the edges of the envelopes on a flat surface.
Gently load the stack of envelopes with the address side facing up
and the flap side come the leftward. Be sure to insert the envelopes
completely.
Be sure that the stack of envelopes is loaded under the paper limit guides.
Paper limit guide
2-16 Loading Paper