Dell C1765NFW MFP Laser Printer - Manual (Page 223)

Loading Print Media 221
Loading an Envelope in the Priority Sheet Inserter (PSI)
NOTE: Be sure to fully insert the envelope until it stops. Otherwise, print media that is loaded on the MPF will be fed.
NOTE: When you print on envelopes, be sure to specify the envelope setting on the printer driver. If not specified, the print
image will be rotated 180 degrees.
When Loading Envelope #10, DL, or Monarch
Load envelopes with the side to be printed on facing up, the flap side facing down, and the flap turned to the right.
To prevent DL and Monarch from being wrinkled, they are recommended to be loaded with the print-side facing up,
flap opened, and facing you.