Brother Copier/Printer MFC-9970CDW - Manual (Page 153)

Troubleshooting
137
B
Envelope creases  Make sure that the two gray envelope levers are pulled down when
you are printing envelopes.
 If creases occur with the envelope levers in the envelope position,
print with the back cover opened and feed one envelope at a time
from the MP tray.
Examples of poor print
quality Recommendation
ABCDEFG
EFGHIJKLMN