The investigations vary widely in scope, in scale, and in complexity. Certainly it depends on the size and the sophistication of the business involved, and on the number of emails sent. All of those factors will play into the appropriate enforcement response. As Steven mentioned, one of the tools we have available is a warning letter to make clear where there are alleged violations and to provide a bit of guidance to companies to help them improve their practices.
On September 26th, 2017. See this statement in context.