Marketers have had tools to answer these questions for a long time. Sidekick, a project from HubSpot, brings this kind of email tracking to one-on-one email communication. For HubSpot, it's a little outside their wheelhouse. Over the last nine years they’ve developed some of the largest and most popular software for marketers. Sidekick is part of a decision to broaden their scope and serve non-marketers. And it's built on the freemium model. "The whole idea from the beginning was to go for really big scale that millions of people could use for free," says Dan Wolchonok. "And then to have a seamless path for those who want to pay for power features." Dan is a senior product manager at HubSpot. It's his job to get those millions of users. Which is less about finding them and more about keeping them around.