The Brother MX-4000 optional mailbox unit helps increase control over your Brother printer's output. Assign the four output bins to specific users or group and send print jobs directly to each recipient’s bin. When printing large documents, use the bins to increase your machine’s output tray capacity. When the output tray is full, the machine automatically delivers the pages into the mailbox bins. Removing bins 2 and 4, increases the machine’s maximum output capacity to 1,050 sheets (including the standard output tray). When printing multiple copies, set the printer driver to deliver each copy into a different bin for higher efficiency. For Use With: HL-L6400DW, HL-L6400DWT, HL-L6400DWG, HL-L6400DWX Unit Dimensions (W" x D" x H"): 14.4" x 16.4" x 15" Unit Weight: 9.7 lbs.