Canon MF244dw User Manual page 56

Basic Operations
4
Align the edges of the envelope on a flat surface.
Loaded Orientation
Load the envelopes in portrait orientation, short edge first, with the non-glued side (front side) face up.
Load envelopes in the same orientation for both the paper drawer and manual feed slot.
Load the envelopes so that their flaps are at the left.
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