Hi haydar_alrikabi,

Please try the below mentioned steps:

1. Press the “Menu” Button on the printer’s operator panel.
2. Use the left/right arrows and select “Adjust Fuser”.
3. Use the up/down arrow and select “Envelope” from the list and select it.
4. Now decrease the temperature using the arrows.
5. When the printed envelope has curled, try to lower the temperature.
6. And now check printing an envelope.

Some tips which help when printing on envelope.

• Use only high-quality envelopes that are designed for use in laser printers.
• Set the print media source to MPF or PSI. Set the paper type to Envelope, and select the correct size of envelope from the printer driver.
• For best performance, use envelopes made from 75 g/m2 (20 lb bond) paper or up to 105 g/m2 (28 lb bond) weight for the envelope feeder as long as the cotton content is 25% or less. Envelopes with 100% cotton content must not exceed 90 g/m2 (24 lb bond) weight.
• Use only freshly unpackaged, undamaged envelopes.
• Use envelopes that can withstand temperatures of 205°C (401°F) without sealing, excessive curling, wrinkling, or releasing hazardous emissions.
• Adjust the guide to fit the width of the envelopes.
• To load envelope in MPF or PSI, the print side must be facing up.
• Use one envelope size during a print job.
• Ensure that the humidity is low because high humidity (more than 60%) and the high printing temperatures may seal the envelopes.
• For best performance, do not use envelopes that:
  • Have excessive curl or twist .
  • Are stuck together or damaged in any way .
  • Contain windows, holes, perforations, cutouts, embossing .
  • Use metal clasps, string ties, or metal folding bars .
  • Have an interlocking design .
  • Have postage stamps attached .
  • Have nicked edges or bent corners .
  • Have rough, cockle, or laid finishes.

Try the above steps and reply for further help.