Brother DCP-8020 - Manual (Page 91)

USING THE DCP AS A PRINTER 6 - 14
4
Pull out the MP tray support flap.
5
Put the envelopes in the MP tray so that they touch the back of
the tray. Do not put more than 3 envelopes in the MP tray at any
one time, or it may cause a jam.
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.
Make sure that the envelopes are neatly stacked and in the
proper position in the MP tray. If they are not, the envelopes may
not be fed properly, resulting in a skewed printout or a jam.