Dell P1500 Owner's Manual page 38

• envelopes, insert them vertically, flap side down with the stamp area in the top left
corner
To reduce paper jams, do not use envelopes that:
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3 8 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