Dell E525w User Manual page 172

To prevent DL and Monarch from being wrinkled, they are recommended to be loaded with the flaps opened. Place
the print side facing up, and the flaps come at the bottom when you face towards the printer.
When Loading C5
Load envelopes with the flaps opened. Place the print side facing up, and the flaps come at the bottom when you
face towards the printer.
CAUTION:
Never use envelopes with windows, or coated linings. These lead to paper jams and can cause damage to the
printer.
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Loading Print Media