Brother MFC 9970CDW - Manual (Page 158)

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Envelope creases  Make sure that the two grey 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