The co-founder and CEO of Slack is predicting an end to the world of email as we know it inside companies in the next seven years.

"Inside our companies, I think that's happening faster and faster. Over the next few years, certainly over the next five to seven years, we'll see a faster change," Slack chief Stewart Butterfield said. But he added, "The broader world of email will stick around."

Slack's paying customers total over 95,000 — with more than 10 million daily active users, the company said in a regulatory filing.

Conventional email will phase out for those users between five to seven years from now, Butterfield estimated.

"Everyone will choose this," he said of his platform, where companies can set up both public and private channels for employees to collaborate and direct message each other.

Butterfield spoke with CNBC's Andrew Ross Sorkin on "Squawk Box," ahead of the company's stock debut Thursday on the New York Stock Exchange.

Slack's listing is different from the slew of tech IPOs this year, including Uber, Lyft, Pinterest and Chewy.

Like music streaming service Spotify, Slack decided to pursue a so-called direct listing, rather than a traditional initial public offering.

Direct listings allow a company to go public without involving underwriters — those intermediaries who buy shares from the company or insiders and then sell them to the public. Instead, the shares simply begin trading on an exchange.

The NYSE has set a "reference price" of $26 per share for Slack, based roughly on the price of private trades over the last few months.

— CNBC's Bob Pisani contributed to this report.