Dell C1660w Color Laser Print User Manual page 164

When Loading C5
Load envelopes with the side to be printed on facing up, with the flap open.
CAUTION:
Never use envelopes with windows, or coated linings. These lead to paper jams and can cause damage to the
printer.
NOTE:
If you do not load envelopes in the PSI right after they have been removed from the packaging, they may bulge. To avoid
jams, flatten them as shown below when loading them in the PSI.
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Loading Print Media