Epson CX6500 Reference Manual page 137

Flatten the envelope flaps before loading.
Flatten the loading edge of the envelope before loading.
Avoid using envelopes that are too thin as they may curl during printing.
Do the following if paper cannot be fed properly.
Feed envelopes with the flap side on the right side and select the Rotate 180° checkbox on
the Page Layout menu (Windows) or the Rotate by 180 degrees checkbox in the Page
Setup dialog box (Macintosh).
Press the four corners of the envelopes firmly with your fingers to make the envelopes flat.
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