mRNA decapping enzyme that specifically removes the nicotinamide adenine dinucleotide (NAD) cap from a subset of mRNAs by hydrolyzing the diphosphate linkage to produce nicotinamide mononucleotide (NMN) and 5' monophosphate mRNA. The NAD-cap is present at the 5'-end of some mRNAs and stabilizes RNA against 5'-processing. Has preference for mRNAs with a 5'-end purine. Catalyzes the hydrolysis of a broad range of dinucleotide pyrophosphates