HP LaserJet 1320 - Manual (Page 61)

Printing an envelope
Only use envelopes that are recommended for laser printers. See Printer media
specifications for more information.
Note You must load envelopes one at a time in the single-sheet priority input slot (Tray 1).
1. To prevent the envelope from curling as it prints, open the straight-through output door.
2. Before loading the envelope, open the single-sheet priority input slot (Tray 1), and adjust
the media guides to the width of the envelope.
ENWW Printing an envelope 51