The claim is exactly the same one Microsoft faced 20 years ago.

Slack filed a complaint with the European Commission detailing "Microsoft's illegal and anti-competitive practice of abusing its market dominance to extinguish competition in breach of European Union competition law", the company said.

The complaint centres on Microsoft Teams, the company's chat and video conference platform. Teams is a competitor product not only to Slack but also to popular conference service Zoom, Google's Meet and chat services, and other video services. Slack alleges that the way Microsoft bundles Teams into its distribution of Office—widely used enterprise software such as Outlook, Word, PowerPoint, and Excel—gives Microsoft an unfair advantage against the competition.

Slack's general counsel, David Schellhase said that Teams needs to be separated from the Office Suite and sold separately with a fair commercial price tag, so it competes on its own merits with its products.

"Competition and antitrust laws are designed to ensure that dominant companies are not allowed to foreclose competition illegally."

He claimed that Vole was reverting to past behavior.

“They created a weak, copycat product and tied it to their dominant Office product, force installing it and blocking its removal, a carbon copy of their illegal behavior during the ‘browser wars."

Slack is asking the European Commission to take swift action to ensure Microsoft cannot continue to illegally leverage its power from one market to another by bundling or tying products.

The European Commission does not necessarily have to investigate Microsoft just because Slack has filed a complaint. Based on the EC's current strong interest in probing alleged anticompetitive behavior from tech companies such as Apple, Google, and Facebook, however, the commission seems primed to take the accusations seriously.

"They want to kill us, as opposed to have a great product and make customers happy", Slack CEO Stewart Butterfield told the WSJ.