Canon PIXMA MP280 Series Advanced Manual page 41

Do not raise the Paper Support.
Open the Paper Output Tray gently, and open the Output Tray Extension.
(2)
Slide the Paper Guides (A) to open them, and load the envelopes in the center of
(3)
the Rear Tray WITH THE ADDRESS SIDE FACING YOU.
The folded flap of the envelope will be faced down on the left side.
Up to 10 envelopes can be loaded at once.
Slide the Paper Guides (A) to align them with both sides of the envelopes.
(4)
Do not slide the Paper Guides too hard. The envelopes may not be fed properly.
(B) Rear side
(C) Address side
Note
Do not load envelopes higher than the Load Limit Mark (D).
3.
Specify the settings with the printer driver.