Shares of Microsoft Corporation (MSFT) might be trading near tech-bubble highs, but leaving the stock out of your portfolio for 2017 continues to be a mistake, according to many Wall Street analysts – some of whom believe the company's Azure cloud generated revenue of $3 billion in the most recent quarter.

Microsoft shares closed Friday at $65.78, just before netting yet another all-time high at $65.91. The cloud-first strategy ushered in by CEO Satya Nedalla is driving the bullishness. Microsoft's Azure Cloud platform is considered "real" cloud computing, according to Barron's. While Azure has grown to become second only to, Inc.'s (AMZN) dominant AWS, Microsoft doesn't break out revenue for the business. Instead, the company reports only the total for its Commercial Cloud. (See also: Microsoft Goes From Window(s) Cleaning to Cloud Gazing.)

Still, the quick rise of Azure and the rate at which corporations continue to adopt the platform bring intrigue to Microsoft shares, especially since the company, which crushed Wall Street's fiscal second quarter revenue and earnings estimates last week, remains secretive about the platform. Bernstein analyst Mark Mordler estimates that Azure generates around 20% of Commercial Cloud revenue, which Microsoft said was recording an "annualized run rate" just north of $14 billion, according to Barron's.

During the fiscal second quarter, Azure revenue surged 93% year over year (up 95% in constant currency). "Our customers are seeing greater value and opportunity as we partner with them through their digital transformation," said Nadella in a statement. "Accelerating advancements in AI across our platforms and services will provide further opportunity to drive growth in the Microsoft Cloud." (See also: Alphabet, Microsoft Earnings Highlight Focus on AI, Products.)

Microsoft estimates Azure compute usage more than doubling year over year. Assuming Mordler's estimates for Azure are correct, in the low 20 percentage range of total Commercial Cloud revenue of $14 billion, this would put Azure revenue at around $3 billion. In other words, Microsoft's Azure business alone is already bigger than a handful of S&P 500 companies.

Why is this important? Pacific Crest analyst Brent Bracelin forecasts that public cloud spending, currently around $58 billion annually, will triple to $205 billion in the next three years. During that span, Bracelin expects spending on software as a service (SaaS) to surge 20% annually, reaching $129 billion by 2020. And the rate at which Azure is growing suggests that Microsoft is well positioned to secure large chunks of that pie. (See also: Microsoft Stock Among Top Cloud Picks for 2017.)