Brother MFC-8420 User Manual page 181

For MFC-8420:
2
Open the back output tray.
3
Open the manual feed tray. Slide the paper guides to fit the
envelope size.
If envelopes are creased after they have been printed:
At the back of the MFC, open the back output tray and push
down the blue tabs at the left and right hand sides (as shown in
the figure on the next page). When you have finished printing
your envelopes, close the back output tray to reset the two blue
tabs back to their original position.
11 - 13 USING THE MFC AS A PRINTER