You can print on plain paper envelopes in this size:
No. 10 (4.1 × 9.5 inches [105 × 241 mm])
Extend the output tray and open the paper stopper.
Pull up the rear MP tray.
Flip the feeder guard forward, then slide the edge guides outward.
Insert envelopes, short edge first and with the flap facing down, in the center of the tray.
Slide the edge guides against the envelopes, but not too tightly.
Always follow these paper loading guidelines:
If you have trouble loading a stack of envelopes, press each envelope flat before loading it or load one envelope at a time.
If print quality declines when printing multiple envelopes, try loading one envelope at a time.
Do not load envelopes that are curled, folded, or too thin.
Loading Paper in the Cassette
Loading Paper in the Rear MP (Multipurpose) Tray
Paper Loading Capacity