Canon LBP7210Cdn Manual page 252

Make sure that the paper stack does not exceed the load limit mark lines (B).
IMPORTANT
Do not load the envelope stack which exceeds the load limit mark lines.
Be sure that the paper stack does not exceed the load limit mark lines. If the envelope stack
exceeds the load limit mark lines, this may result in misfeeds.
Insert the paper drawer into the printer.
Push it into the printer firmly.
CAUTION
Precautions when inserting the paper drawer
Be careful not to catch your fingers.
Step 2: Registering the Size of Paper