Brother MFC-9130CW Basic User's Manual page 156

Examples of poor print quality
Envelope Creases
142
Recommendation

Make sure the envelope levers inside the back cover are
pulled down to the envelope position when you print
envelopes (see Loading paper in the manual feed slot
on page 22).
NOTE
When you have finished printing, open the back cover and
reset the two gray levers back to their original positions (1).
Pull up the two gray levers until they stop.
1