the base unit 12 beneath the lower run 40 of the belt 34 each roller assembly 42 having a spring loaded arm 44 pivotally mounted on the base unit 12 and carrying a back up pressure roller 46 adjacent the free end of the arm 44 with this arrangement the plurality of back up rollers 46 maintain an envelope in firm driving engagement with the lower surface of the lower run of the belt 34 since the conveyor assembly 30 forms no part of the present invention further description thereof is not deemed necessary for a full understanding of the present invention with particular reference to fig2 and 3 it will be seen that the ink jet printing assembly 32 is mounted in the mailing machine base 12 generally in a location toward the downstream end of the envelope conveyor 30 and spaced therefrom toward the rear portion of the mailing machine 10 the printing assembly 32 includes at least one but preferably a pair of digital ink jet print heads 48 and 50 which are suitably mounted on a housing 52 the housing 52 is suitably mounted on a frame 54 which in turn is mounted for lateral movement within the mailing machine base 12 between an intermediate position and two extreme positions the printing assembly 32 being.