Canon MX390 series Online Manual page 42

Note
Do not overload envelopes by stacking until the top of the stack touches the paper guides.
(5) Open the output tray extension.
3. Open the paper output tray.
(1) Open the paper output tray gently, and extend the paper output support.
Important
Be sure to open the paper output tray when printing. If not open, the machine cannot start
printing.
Note
After loading envelopes
Select the size and type of the loaded envelopes on the print settings screen of the printer driver.