Epson AcuLaser C8600 Reference Manual page 48

The following table shows important settings you have to make
when using envelopes:
Paper Source
SelecType Tray Menu
Printer driver settings
Please note the following tips for handling this media:
Load envelopes with the flap open and facing outward as
shown below.
46 Paper Handling
setting in the
MP tray only (up to 20 envelopes)
, , , ,
Paper Size: Mon, C10, DL, C6, C5
Paper Source: MP tray
Paper Type: Thick