Canon LBP6230dw Manual page 37

Loading Envelopes in the Multi-Purpose Tray and Manual Feed Slot
Load the envelopes Monarch, No. 10 (COM10), DL, or C5 in portrait orientation (with the short edge toward the machine), with the non-
glued side (front side) face up. You cannot print on the reverse side of envelopes.
When loading in the manual feed slot, only one envelope can be loaded at a time.
Load the envelopes so that the edge with the flap is toward the left side as shown in the illustration.
Multi-purpose tray
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Manual feed slot