In 2002 the European Union (EU) introduced the Directive on Privacy and Electronic Communications. Article 13 of the Directive prohibits the use of personal email addresses for marketing purposes. The Directive establishes the opt-in regime, where unsolicited emails may be sent only with prior agreement of the recipient; this does not apply to business email addresses.
The likely answer is to do with audience biases. A survey conducted by an email marketing provider is almost certainly going to have different results to one conducted by PPC management tool, as their audiences have different skillsets and biases, skewing the results of their sample. As such, we should take the specific ranking of different strategies in these studies with a pinch of salt.
The inbound method attracts leads using online content. You create a website, or you write a blog, and you optimize it for online search through SEO (search engine optimization) techniques. This means the content has the appropriate keywords and answers the questions your target audience is asking. When your content is easily discoverable and begins to engage your readers, they become your leads. Depending on how you interact with them from then on, they can become your customers too. https://www.sendblaster.com/wp-content/uploads/2018/03/your-email-marketing-campaign-in-minutes.png