In this paper we present a fast approach for the detection of address blocks on images of oversized flat mail--pieces with a size up to 35x25 cm. In contrast to standard size envelopes the position of the address block cannot be restricted to the lower right region of the mail--piece. Furthermore the envelopes are mixed up with advertizing, the return address, logos and other noise making the detection more complicated. In addition, for usage in a practical mail sorting system the recognition of the address block has to be accomplished within 200 ms for handwritten and machine printed addresses.In this approach a method based on the detection of inhomogeneous blocks is described. The image of the envelope is divided into small rectangular blocks for which a measure of homogeneity is evaluated. After determining an adaptive threshold inhomogeneous blocks are detected in a hierarchical search process. Adjacent inhomogeneous blocks are then merged to areas of arbitrary shapes. These areas are represented by the smallest enclosing rectangle. Several features are calculated for each area and a plausibility test is performed. The best scored area is assumed to contain the address block and is passed to an optical character recognition system. This approach meets the above mentioned time constraint on an i--860 based computer and has an excellent reliability.