Canon IR1600 Reference Manual page 51

Loading envelopes in the stack bypass
Place the envelopes on a level surface and press their edges flat.
Take the envelopes and loosen them as shown, then stack them together.
Load the envelopes as shown below (when printing on the upper side).
IMPORTANT
Make sure that the envelopes are not inflated with air. Press the stack of envelopes to empty the envelopes.
You cannot print on the back side of the envelopes.
Depending on the paper type or condition of the envelopes, the printing quality may deteriorate or paper jam may
occur. In such a case, it is recommended to change the type of envelopes.
Making Prints Using the Stack Bypass
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