Slack has filed an antitrust complaint against Microsoft, accusing it of blocking competition with its Teams app

• Slack said on Wednesday that it had filed a complaint against Microsoft with the European Commission over its Teams chat software.
• The complaint accused Microsoft of engaging in anticompetitive practices by tying its Teams office-chat app to its popular Office 356 productivity suite.
• Major tech firms like Apple, Amazon, Google, and Facebook have been scrutinized over antitrust concerns, which Microsoft had largely dodged until now.
• Microsoft is Slack's biggest competitor in the work-communication software space, and both have grown throughout the COVID-19 pandemic as more people have been working remotely.

Slack, the workplace-communication software maker, said on Wednesday that it had filed a competition complaint against Microsoft with the European Commission over its Teams software, a move that comes as tech giants have faced increased scrutiny over their size and influence on the market.

Slack accused Microsoft of engaging in anticompetitive practices by tying its Teams chat-and-collaboration app — which directly competes with Slack — to its popular Office 365 productivity suite.

"We're confident that we win on the merits of our product, but we can't ignore illegal behavior that deprives customers of access to the tools and solutions they want," Jonathan Prince, the vice president of communications and policy at Slack, said in a press release. "Slack threatens Microsoft's hold on business email, the cornerstone of Office, which means Slack threatens Microsoft's lock on enterprise software."

The European Commission will have to review Slack's complaint before deciding whether to launch an investigation into Microsoft.

Microsoft's Teams app is Slack's biggest competitor in the workplace-communication software space. Both companies boomed during the coronavirus pandemic as businesses around the world were forced to shift to remote-work arrangements. Microsoft said in March that it had added 12 million daily active users for Teams in a single week, while Slack said at about the same time that it had added 7,000 new paid customers since February. (Slack has not released daily-active-user figures since October, when it had 12 million.)

"Teams and Slack are in a fierce battle for market share," the Wedbush Securities analyst Dan Ives previously told Business Insider's Paayal Zaveri, adding: "This trend is accelerating and is probably going to continue to accelerate over the coming months as more employees around the world are working remote."

Microsoft was the subject of a historic antitrust battle in the late 1990s, when Judge Thomas Penfield Jackson ruled that its dominance of the PC market deemed it a monopoly. But it has dodged the mounting scrutiny that other large American tech companies like Apple, Amazon, Google, and Facebook have faced in recent years.

The CEOs of these companies are set to testify before Congress next Monday about possible antitrust practices.

Brad Smith, Microsoft's president, has recently been outspoken about the importance of facilitating competition in the tech industry when it comes to app stores. He was tapped by the House of Representative's antitrust subcommittee to share his perspective ahead of the hearing, given Microsoft's experience, according to reports from Bloomberg and The Information.

Smith is said to have raised concerns in that meeting about the way Apple, a Microsoft competitor, manages its App Store, which has faced accusations of anticompetitive behavior.

"The time has come, whether we are talking about DC or Brussels, for a much more focused conversation about the nature of app stores, the rules that are being put in place, the prices and the tolls that are being extracted and whether there is really a justification in antitrust law for everything that has been created," Smith said during a livestreamed Politico event on June 18.

Correction: An earlier version of this article incorrectly referred to the number of new customers Slack added between February and March. It added 7,000 new paid customers, not 7 million new users.