Slack, the cloud-based collaboration tool used by more than eight million people across thousands of companies, is folding an email client into its ever-expanding software portfolio. The San Francisco company announced today that it’s purchased Astro, a messaging startup that leverages artificial intelligence (AI) to supercharge inboxes.

Palo Alto-based Astro’s 28-person team will join Slack later this year, making it the latter’s biggest acquisition to date. In a blog post on its website, Astro said it plans to shut down its apps for Mac, iOS, Android, Amazon Alexa, and Slack on October 10.

Astro’s platform, which integrated with Office 365 and Gmail, offered a standard array of features including snoozing, reminders, and a priority inbox that tapped algorithms to surface key threads. But the headliner was Astrobot, a chatbot that made recommendations — like offering to archive recurring emails that you frequently ignore, for example, or reminding you to follow up with specific contacts — informed by your behavior, and enabled universal search across email, calendars, and Slack conversations.

“Slack is the maker of the fastest-growing enterprise app in history, and we’re joining them to help connect email and calendaring to all the work teams do in Slack,” Andy Pflaum, CEO of Astro, said. “[A]s we explored with Slack how to bring together messaging, email & calendar, it became evident that we would have the biggest impact on workplace communications and realize our original vision by joining Slack.”

In January, Pflaum told VentureBeat that its user base was in the “six figures.”

“We’ve taken some steps to make it possible to integrate email into Slack, but now we’re in a position to make that interoperability much simpler and much, much more powerful,” Slack said in a statement. “Our goal is to make it as easy as possible to help teams shift conversations to where they would be most productive — in a channel, alongside the relevant context and software tools teams use at work, from ServiceNow and Salesforce to Workday and Box.”

The Astro acquisition comes a little less than two months after Slack snapped up Hipchat and Stride, two workflow productivity apps designed for large enterprises, from Sydney, Australia-based company Atlassian. As part of a “strategic partnership,” Slack agreed to pay an undisclosed amount over the next three years for the intellectual property underlying Hipchat Cloud and Stride.

In July, it acquired Missions, a Robots & Pencils startup focused on process automation and streamlining custom Slack integrations.

More than 70,000 teams — or roughly three million individual users — pay for Slack. (That’s compared to the 329,000 organizations that use Microsoft Teams and 30,000 businesses using Facebook’s Workplace.) It raised $427 million in a series H funding last month from Dragoneer Investment Group, General Atlantic, and others, giving it a valuation of about $7 billion.