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This article presents new upper and lower bounds on the symmetric cutoff rate for block-coded quadrature-amplitude-modulated (QAM) signaling over links affected by time-correlated Rayleigh-distributed flat-faded phenomena. The proposed bounds assume maximum-likelihood soft-decoding with perfect channel-state-information at the receiving side and hold for any form of QAM constellations. These bounds are quickly computable and constitute an efficient means to estimate the cutoff rate of systems employing very long codewords, so that an exact evaluation of the cutoff rate results. Analytical and numerical evidence of the tightness of the presented bounds is also provided.