Dell P1500 Owner's Manual page 46

To reduce paper jams, do not use envelopes that:
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Slide the paper guides flush against the edges of the paper or envelope.
6
NOTE: Do not forget to select the paper type and paper size from your software application.
4 6 Lo a d p ap e r
Have excessive curl
Are stuck together
Are damaged in any way
Contain windows, holes, perforations, cutouts, or embossing
Use metal clasps, string ties, or metal folding bars
Have postage stamps attached
Have any exposed adhesive when the flap is in the sealed position