Shares of Slack skyrocketed after word leaked out that Salesforce was in discussions to acquire the company. If the deal is completed, it would be Salesforce’s biggest to date. The transaction would join the business software behemoth with the channel-based messaging platform that could take the place of office email.

Slack shares of closed nearly 38% higher on Wednesday after the Wall Street Journal first reported that Salesforce had been holding negotiations to acquire the company.

Salesforce stock closed down 5% on Wednesday. That company will release its fiscal third-quarter earnings next week.

It’s not known how much the deal would be worth, but Slack’s market cap ballooned to more than $20 billion following the report.

Salesforce on Spending Spree

Salesforce has enjoyed an acquisition spree, leveraging its market cap growth in the past few years to acquire growing companies. In 2018, Salesforce bought MuleSoft for $6.5 billion. That was the company’s largest deal ever at the time. The MuleSoft acquisition aimed at connecting cloud applications. MuleSoft software connects data stored in the company’s data centers and data hosted elsewhere.

Salesforce then spent more than double that to get Tableau. In 2019, it acquired the data visualization company for $15.3 billion. Tableau isn’t a cloud company, as its products run primarily in on-premises data centers. But more than one-third of deployments are in the cloud, according to Tableau’s CEO Adam Selipsky on the call with analysts after the deal was announced.

At the time, Salesforce CEO Marc Benioff called Tableau a “jewel in our industry” and said “our customers are looking to do a lot more in this area” as far as integrating the software across Salesforce’s products.

The acquisition of Slack would be one of the largest software deals ever in the industry. Observers say that the price tag would rank among IBM’s $34 billion purchase of Red Hat in 2019, Microsoft’s $27 billion purchase of LinkedIn in 2016, and Facebook’s $19 billion purchase of WhatsApp back in 2014.

And in February of this year, Salesforce signed a definitive agreement to acquire Vlocity. The deal closed in June. Built natively on the Salesforce platform, Vlocity is a provider of industry-specific cloud and mobile software for communications, media and entertainment, energy, utilities, insurance, health and government organizations.

What About Microsoft?

Microsoft is already a competitor of Salesforce in software for tracking customers. TechCrunch reported in 2016 that Microsoft was considering the purchase of Slack for up to $8 billion, but no deal was struck. A few month later, Microsoft introduced the Teams communication app.

“Our primary competitor is currently Microsoft Corporation,” Slack said when it was looking to go public in 2019. Microsoft has a large customer base it’s been able to convert many of them to Teams. Salesforce could provide Slack with a similar benefit.

Founded in 2009 in Vancouver, British Columbia, Canada, Slack last reported in October 2019 that it had more than 12 million daily active users. In March, Slack saw its concurrent users surpass 10 million. It then jumped to 10.5 million just six days later on March 16th. Ten days later it had increased to 12.5 million. Slack has not announced its total daily active users during this period—just simultaneously connected users.

By comparison, Microsoft Teams usage has soared over the past year, reaching 44 million daily active users during a big increase in demand in March as the coronavirus pandemic hit. In October, Microsoft reported that its Teams usage jumped 50% to 115 million daily active users.

Salesforce was added to the Dow Jones Industrial Average on August 31, 2020.