Xerox Phaser 7800DN User Manual page 54

Paper and Media
Loading Envelopes in Tray 1
1. Open Tray 1 and remove any paper that is a different size or type.
2. Move the width guides to the edges of the tray.
3. Load the envelopes with the flaps closed and facing down:

Load envelopes up to 320 mm (12.6 in.) long-edge feed, with the flaps leading into the printer.
54 Phaser 7800 Color Printer
User Guide