HP Color LaserJet CM1015CM1017 Multifunction Printer series - Manual (Page 67)

4. Slide the media-width guides slightly wider than the print media.
NOTE Load a maximum of 10 envelopes into tray 2 with the flaps on the right side, the
side to be printed facing up, and with the short edge leading into the MFP. If the envelope
has a flap on a short edge, this edge must enter the MFP first.
ENWW Load trays 53