Epson EPI-4000 User Manual page 111

Loading Paper
4. If you want to print a single sheet, make sure the envelope lever
is in the single-sheet position, as shown below. To print an
envelope, move the envelope lever to the envelope position.
CAUTION: If the envelope lever is in the envelope
position when you are using single sheets, the printer
does not load paper or print properly.
5. Close the paper feeder cover if you want to print a single sheet.
To print an envelope, keep the paper feeder cover open.
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Paper Handling