Brother HL-2400C Service Manual page 392

USER'S GUIDE
3–12
Before loading envelopes in the cassette, check the following:
• Envelopes should have a lengthwise sealing flap.
• The sealing flaps should be crisply and correctly folded (irregularly
cut or folded envelopes may cause paper jams).
• Envelopes should consist of two layers of paper in the following
marked area.
Feeding Direction
Fig. 3-4 Envelopes
• Envelope joints that are sealed by the manufacturer should be secure.
• All sides should be properly folded without any wrinkles or creases.
• Print quality may vary between different style envelopes. It may be
necessary to test the envelopes you wish to use prior to purchasing
large quantities.