Dell 1350 Color User Manual page 175

When Loading C5
Load envelopes with the side to be printed on facing up, with the flap open.
NOTICE:
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 MPF 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 MPF.
Loading Print Media
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