Sales Tax on Adobe Sign

I am located in Massachusetts and noticed our annual invoice for Adobe Sign Adobe didn't charge us sales tax. We also have never provided Adobe an Exempt Certificate. 

Since I underestood that Adobe always charges tax it seems that it has been determined that Adobe Sign is not taxable in the Mass. If so, is there a letter Ruling from the Mass DOR why the tax is not charged, and if not, is there a rationle why Adobe did not charge the tax?

Thanks