Please check the following points to solve the problem:

NOTE: Illustrations shown below are from a representative product, and may differ from your Brother machine.

1. Make sure that the envelope levers inside the back cover are pulled down to the envelope position when you print to the envelope.
   
   
   
    
   
   
2. 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.