Canon MG6400 SERIES Online Manual page 121

(5) Slide the right paper guide
of the envelopes.
Do not slide the paper guides too hard against the envelopes. The envelopes may
not be fed properly.
Note
Do not load envelopes higher than the load limit mark (D).
Keep the envelope stack height below the tabs
(6) Holding the right tab on the front tray, push the feeder back until the
arrow ( ) on the front tray is aligned with the arrow ( ) on the machine.
(A) to align the paper guides with both sides
(E)
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of the paper guides.