In a big win for Slack, Business Insider (BI) is reporting that IBM is deploying the messaging app to all of its 350,000 employees.

Slack is locked in a rivalry with Microsoft Teams, with the two companies battling for the corporate messaging market. Microsoft Teams recently doubled Slack’s user base, and has kept the pressure up with TV ads. In spite of Microsoft’s momentum, however, IBM has chosen Slack as its messaging app of choice. This, in turn, helps Slack make the case that it can compete with Microsoft on the largest scale, in the most mission-critical environments.

“Going wall to wall in IBM — it’s basically the maximum scale that there is, so we now know that Slack will work for literally the largest organizations in the world,” Slack CEO Stewart Butterfield told BI.

Konrad Lagarde, director of IBM Toolbox, told BI that one of the reasons IBM went with Slack was their willingness to meet IBM’s needs. When Lagarde first starting using the app, teams were limited to 2,000 individuals. With some departments larger than that, IBM needed an app that could scale better and Slack was willing to add the necessary features.

IBM also likely chose Slack over Microsoft Teams as a result of increasing competition between the two computing giants. Microsoft is second in the U.S. cloud market, and IBM has increasingly staked its future on moving into the cloud. In fact, IBM’s recent earnings were buoyed by its cloud business. Just as many retail companies are turning to Microsoft rather than relying their prime competitor Amazon, IBM probably wants to avoid relying on a company it directly competes with.

Either way, today’s announcement is good news for Slack and will likely help the company continue to attract business, both large and small.