Brother HL-6050 User Manual page 35

If envelopes get smudged during printing, in the printer driver select MP Tray as the Paper
Source for a straight paper path. Then, set the Media Type to Thick Paper or Thicker Paper
to increase the fixing temperature. To adjust the print density setting, See QUALITY on page
3-15.
Envelope joints that are sealed by the manufacturer should be secure.
All sides should be properly folded without any wrinkles or creases.
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