Brother MFC-9140CDN User Manual page 164

Examples of poor print quality
Envelope Creases
154
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 23).
NOTE
When you have finished printing, open the back cover and
reset the two grey levers back to their original positions (1).
Pull up the two grey levers until they stop.
1