Dell 1250 Color User Manual page 67

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.
NOTE:
If envelopes are still not fed correctly, add some bending to the flap of the
envelopes as shown in the following illustration.
The amount of the bending shall be 5mm (0.20 inches) or less.
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