Epson Stylus COLOR 660 Daily Use page 17

Load envelopes flap edge first with the flap side down, as
shown in the illustration.
Do not use envelopes that are curled or folded. Make sure to
flatten the envelope flaps before loading. Also avoid using
envelopes that are too thin; they may curl during printing.
Select Plain Paper as the Media Type setting on the printer
driver.
Available envelopes sizes are:
No.10, DL, C6
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