Brother MFC-J1010DW Online User's Manual page 46

5. Gently press and slide the paper width guides (1) and paper length guide (2) to fit the size of the envelopes.
Make sure the envelopes are flat in the tray.
1
2
IMPORTANT
If envelopes are multi-feeding, load one envelope in the paper tray at a time.
6. Close the output paper tray cover.
7. Slowly push the paper tray completely into the machine.
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