According to Yahoo (YHOO) Advertising Solutions, 78 percent of chief marketing officers think custom content is the future of marketing. In 2013, according to eMarketer, roughly $118 billion were spent on unique content marketing, video marketing, and social media. The services that deliver this material, such as HubSpot, Eloqua, Marketo, and ExactTarget, have exploded in the past few years as more marketers try to boost customer loyalty or nurture sales prospects.

The problem is that most of these services are no more than automated mass marketers, relying on e-mail—the 8-track technology of marketing tools—as their primary means of delivering content. Here’s why that approach is doomed:

1. Millennials consider e-mail passé and do everything they can to avoid it. As the influence of this next generation of decision-makers grows, the value of e-mail communication will decline.

2. The average corporate user spends more than two hours a day reading and responding to an average of 114 e-mails. It’s clear that our inboxes are becoming bloated, inefficient, and unmanageable.

3. E-mail is neither social nor collaborative. The rise of social media has made it painfully obvious just how inefficient e-mail can be when trying to share information or collaborate with colleagues.

A common problem with e-mail marketing is that many people (read “prospects”) are turned off because they are forced to register too early in the process, before they can determine if the material is of interest.

Now turn that around. The natural way people find content today is via their social networks and search engines. As social content management companies, such as HootSuite, PaperShare, and WildFire, increasingly embrace the social media leaders Google (GOOG), LinkedIn (LNKD), Twitter (TWTR), Facebook (FB), Instagram, etc., the value of an e-mail address is likely to diminish.

That is the future of mass marketing. It starts with focusing on the content itself and then on coming up with a strategy to track and measure engagement with that material. By focusing on the content (rather than the delivery mechanism, such as e-mail), individual pieces can be tagged, tracked, and shared anonymously. Standard metrics, such as “click-through rates” from e-mail, will evolve into “engagement rates.”

Not only will this approach be kinder to our inboxes, but it will also prove to be more effective. After all, content is king.