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Slack just landed a huge account, and it's sending its stock price soaring.

Shares of the instant-messaging platform gained as much as 21% on Monday morning when Business Insider's Paayal Zaveri exclusively reported that IBM would deploy the messaging tool to all 350,000 of its employees around the world.

If the intraday high holds, it would be the best trading session since Slack's first day as a public company on June 20. The company tested public markets through an unconventional direct-listing process.

The deal with IBM builds on the companies' relationship. At least some IBM employees have been using Slack since 2014, and the company began a partnership with the messaging platform in 2016, according to the report.

But in late 2019, IBM decided to go "all in" with Slack. IBM was already the platform's largest customer to date.

Read more: Slack just scored its biggest customer deal ever, as IBM moves all 350,000 of its employees to the chat app

It's a huge account to win for Slack, especially as competition heats up in the instant-messaging space. The messaging platform's stock fell as much as 11% in November after Microsoft announced that its rival messaging service, Teams, had 20 million daily active users. At the time, that was 8 million more daily active users than Slack, which had reported in October that 12 million people were on the platform every day.

The addition of all IBM employees gives Slack an immediate boost to daily active users. Slack CEO Stewart Butterfield said that when the app was created, he never thought a company as large as IBM would want to use it. But as IBM employees used and liked the app, it helped Slack expand the product and appeal to other larger customers.

Slack had climbed just 2% year-to-date through Friday's close.

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Clarification: An earlier version of this story said that as a result of its expanded deal with Slack, IBM is now Slack's largest customer. IBM was already Slack's largest customer. The latest deal means that 100% of IBM's employees are on now on Slack.

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