Comment by CuriousNinja

In an ideal world the latter would be best. However in practice systems tend to get complicated over time as they evolve and more features etc.. are added. I think one way to look at it is that as a business to outsourcing the non-differentiated heavy lifting to another entity that has more expertise in it would let you focus on your core products. In this specific example why is using a cloud email provider any different from deciding to use power from the power grid instead of generating your own electricity.