Epson Stylus 10000 Series User Manual page 39

Note:
Do not use envelopes that are too thin; they may curl during
printing.
Return unused envelopes to the original packaging.
Unfavorable storage conditions, especially high temperature,
humidity, and direct sunlight, can damage envelopes.
Do not choose 720 dpi resolution for envelope printing.
10. Re-adjust the paper-thickness lever to its original position
after you finish printing on envelopes or thick paper.
Otherwise, the print on plain paper may be faint.
Choosing and Loading Paper
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