Brother DCP-8020 - Manual (Page 89)

USING THE DCP AS A PRINTER 6 - 12
3
Open the manual feed tray. Slide the paper guides to fit the
envelope size.
4
Using both hands put the envelope in the manual feed tray until
the front edge of the envelope touches the paper feed roller.
Hold the envelope in this position until the DCP automatically
feeds it in for a short distance, and then let go of the envelope.
If envelopes are creased after they have been printed:
At the back of the DCP, open the back output tray and push
down the blue tabs at the left and right hand sides (as shown in
the figure below). When you have finished printing your
envelopes, close the back output tray to reset the two blue tabs
back to their original position.