Epson Action Laser II - Manual (Page 112)

Choosing Paper
Envelopes-Load envelopes face down as shown below. The
printing quality on envelopes may be irregular because different
parts of an envelope have different thicknesses. If printing is too
light, turn the print density knob clockwise. Always return the print
density knob to the center (default) position when you go back to
printing on ordinary paper.
5-4 Paper Handling