Lexmark MS531
Step 1
Use an envelope that has been stored in a dry environment.
Note: Printing on envelopes with high moisture content can seal the flaps.
Send the print job.
Does the envelope seal when printing?
Go to step 2.
The problem is solved.
Step 2
Make sure that paper type is set to Envelope.
From the home screen, touch Settings > Paper > Tray Configuration > Paper Size/Type.
Contact customer support.
Paper Curl Issue when Printing from the Multipurpose Feeder Tray
200.05 Paper Jam When Printing Duplex from MPF