Canon PIXMA MX870 - Manual (Page 117)

The folded flap of the envelope will be faced down on the left side.
Up to 10 envelopes can be loaded at once.
(4)
Slide the Paper Guides (A) to align them with both sides of the envelopes.
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.
(1)
Select Envelope in Commonly Used Settings on the Quick Setup sheet.
(2)
Select DL Env. or Comm. Env. #10 in the Envelope Size Setting window.
(3)
Select Landscape in Orientation.
Important
If you do not specify the envelope size or orientation properly, the address will be printed
upside down or will be turned to 90 degrees.
Page 117 of 1127 pages Loading Envelopes