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In this letter, we consider a multipoint-to-multipoint transmission system which employs a single relay in wireless networks where all source, destination and relay nodes are equipped with multiple antennas. For amplify-and-forward relay systems, we propose new linear processing strategies for maximizing the sum rate performance by applying a blockwise relaying method combined with convex optimization techniques. To reduce a computational complexity, we first consider a zero-forcing based relay only optimization scheme, which generate a closed-form solution in a non-iterative fashion. In order to further improve the sum rate at low signal-to-noise ratio regimes, we additionally design an enhanced relay filter by exploiting the blockwise relaying method based on a minimum mean-square error criterion. Simulation results show that the proposed relay design strategies outperform the existing conventional schemes in terms of the sum rate.