Epson XXX-00 Daily Use page 18

When the adjust lever is in the "+" position, make sure the High
Speed setting in the printer driver's Advanced dialog box is off.
To turn this option off, you may also need to change the Print
Quality setting.
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.
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Available envelopes sizes are:
No.10, DL, C6, 132 mm 220 mm