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Explanation: If this is the heading of a leader, or the address on an envelope like, "Spett. le Società S.p.A.". Then simply put "Società S.p.A.". Otherwise just "Dear" followed by a name or something "Dear Manager".
If the CEO and some of directors of the company are women, then "Dear Sirs" assuming that all management are male makes a terrible impression and is antiquated anyway now. Not PC.