Canon Pixma MX520 Series Online Manual page 48

(3) Load the envelopes in the center of the front tray WITH THE ADDRESS SIDE FACING
DOWN.
The folded flap of the envelope will be faced up on the left side.
(B) Rear side
(C) Address side
Push the envelopes all the way to the back of the front tray.
Up to 5 envelopes can be loaded at once.
(4) Slide the right paper guide
Do not slide the paper guides too hard against the envelopes. The envelopes may not be fed properly.
Note
Do not overload envelopes by stacking until the top of the stack touches the paper guides.
(5) Open the output tray extension.
(A) to align the paper guides with both sides of the envelopes.
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