Brother HL-3450CN Series User Manual page 69

Avoid using envelopes with the following characteristics:
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Glossy or shiny surfaces
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Protection cover on the envelopes' adhesive parts
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Sealing flaps that have not been folded at purchase
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Sealing flaps as shown below
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Three or more layers of paper in the marked area
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Each side folded as shown below
CHAPTER 3 BEFORE WORKING WITH THE PRINTER
Fig. 3-5
3-13