Samsung ML 2550 - Manual (Page 79)

U SING P RINT M ATERIALS 4.16
6 When you print on envelopes, configure the paper source,
type and size properly in the software application. For
details, see page 5.7.
For details about setting on the control panel, see page 3.5.
7 After printing, close the Multi-Purpose Tray and the rear
output tray.