Corporate messaging platform Slack has started the week in great shape. This Monday, the cloud-based company announced its biggest deal to the date: the addition of IBM to its customer portfolio, as reported by Markets Insider. 

American tech giant IBM has been using Slack since 2014 through a few employees, but it was only in 2016 that the company decided to set a partnership with the messaging platform. Now, IBM has decided to bet heavily on Slack, moving all of its 350,000 worldwide employees to the chat app. 

The market reacted positively to the new deal, as Slack’s shares climbed as much as 21% this Monday. If the upper holds, it would be Slack’s best trading session since its market debut as a public company in June, through a direct listing.

ALSO READ: How Microsoft uses technology to empower people and businesses

IBM’s addition comes at a great time for Slack, since the messaging app has been struggling with serious competition against Teams. Microsoft‘s own corporate chat reported 20 million daily active users in November, and such announcement made Slack, which had 12 million daily active users at the time, to fall as much as 11% in the stock market. 

CEO Stewart Butterfield said, according to what Business Insider reported, that when Slack was created, he never thought a company the size of IBM would want to work with it. However, as IBM employees start using and enjoying the app, it helped them expand the product, making it attractive to other big accounts.