Shares of Slack (WORK)  rose more than 3% Monday after analysts at Canaccord, Barclays and William Blair initiated bullish coverage on the messaging service. 

Positive ratings and price targets sent WORK up 3.02% to close at $34.75. Slack went public in June via a direct listing, with a $26 "reference price" and a $38.50 initial trade price.

Canaccord analyst Richard Davis initiated Slack with a buy rating and $40 price target on the belief that the company is building a product that could replace email completely.

"The days when 'You've Got Mail' elicited joy have long since devolved to 'When will I have time to read those 50,000 unread emails?'" Davis wrote in a note. "The rise of communications -- mobile, real-time, contextual and team focused -- has arrived. Slack has the most advanced, easiest to use, and most integrated to other applications real-time chat engine in the market." 

Barclays initiated Slack with an overweight rating and $45 price target as the firm also believes that Slack is positioning itself as a unique upgrade over email. 

"Slack is different from other high growth software businesses we follow, as it is creating its own market, rather than offering a better version of something that is already there," Barclays analyst Raimo Lenschow wrote. "To analyze the opportunity, we first need to understand the size of the market." The firm sees a total addressable market between $18 billion and $29 billion for Slack.

Meanwhile, analysts at William Blair have estimated a total addressable market of $46 billion -- double the $28 billion that Slack itself has estimated. 

However, Microsoft's (MSFT - Get Report) competing product, named "Groups," has been growing rapidly. MSFT is already claiming a larger number of daily users than what Slack has reported.  

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(This article has been updated.)