Postal operators worldwide use two-dimensional Data Matrix bar codes for machine-readable postage payment evidencing. Dynamic interaction of ink jet inks with uncontrolled media like envelope papers generates a variety of print quality defects. Adaptive thresholding algorithms optimized for OCR amplify the impact of these defects on the readability of two-dimensional bar codes. Print quality parameters are determined according to international standards for several combinations of monochrome franking ink jet inks and plain paper envelopes. The ink and paper characteristics relevant in generating the defects are measured and interpreted in terms of wetting and capillarity. We investigate, through computer simulation, the impact of the interaction of print quality defects and OCR adaptive thresholding algorithms on 2D bar code readability.