Brother MFC-8380DN User Manual page 136

Examples of poor print
quality
Envelope creases
Curled or wavy
126
Recommendation
Make sure that the envelope levers inside the back cover are pulled
down when you print the envelopes.
Check the paper type and quality. High temperatures and high
humidity will cause paper to curl.
If you do not use the machine often, the paper may have been in the
paper tray too long. Turn over the stack of paper in the paper tray.
Also, fan the paper stack and then rotate the paper 180° in the paper
tray.