Workplace messaging startup Slack has received inquiries from Amazon about a potential acquisition, Bloomberg reported Thursday. The online retail giant reportedly is just one of several suitors.

A deal with Amazon could give Slack a valuation of at least $9 billion, sources told Bloomberg. The status of the discussions is still unknown, however, and neither company has commented publicly.

Slack, which was Inc.'s company of the year in 2015, raised $200 million in its latest funding round last year, valuing it at $3.8 billion. Launched in 2013, the company reportedly has five million daily active users and $150 million in annual revenue through January.

Slack CEO and co-founder Stewart Butterfield said in May 2015 that he had already received eight or 10 acquisition offers. Microsoft considered offering to buy Slack for as much as $8 billion last year, according to TechCrunch, but ultimately decided against the idea. Last November, Microsoft released a chat platform called Teams aimed at competing with Slack.

Butterfield told Inc. in 2015 that people quit jobs at Uber and Google to work for Slack, believing that it will continue to grow. "Not that the expectation that everyone should make millions of dollars is all on me," Butterfield said, "but the expectation that they didn't make a dumb decision that was harmful to their families is."