Epson Stylus Photo 750 User Manual page 77

If the stack of
envelopes is more
than 0.4-inch
(10 mm) thick, press
the envelope edges
to flatten them before
stacking or load one
envelope at a time.

    
 
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You can load up to 10 envelopes flap edge first, with the printable side up,
as shown below:
When you print on envelopes or other thick media, set the paper
thickness lever to the + position as shown below. (Return the lever to
the 0 position before printing on regular media.)
Select Plain paper as the Media Type and choose the correct envelope
size as the Paper Size setting in the printer software.