Microsoft has banned its employees using Slack, as well as discouraging the use of arch rivals Amazon Web Services (AWS) and Google, according to GeekWire.
The tech site obtained an internal list from the vendor outlining prohibited and discouraged technology.
The tech giant's reasoning for banning employees from using the free version of comms and messaging app Slack is due to IT security and protecting company secrets.
Slack's Enterprise Grid - which was introduced in 2017, a year after Teams was launched - manages to make the grade, although Microsoft still pushes its employees to use Teams.
"Slack Free, Slack Standard and Slack Plus versions do not provide required controls to properly protect Microsoft Intellectual Property (IP)," reads the internal list.
"Existing users of these solutions should migrate chat history and files related to Microsoft business to Microsoft Teams, which offers the same features and integrated Office 365 apps, calling and meeting functionality.
"Slack Enterprise Grid version complies with Microsoft security requirements; however, we encourage the use of Microsoft Teams rather than a competitive software."
Joining Slack on the "prohibited technologies" list is grammar checker Grammarly which Microsoft claims can "access Information Rights Management (IRM)-protected content within emails and documents" on its network, as well as Kaspersky security software.
AWS, which competes with Microsoft's Azure offering, and Google Docs, which rivals its Office 365 product, fall under "discouraged technologies" and require a "business justification" for use, the internal list stated.
"It is highly recommended to start a migration plan to Azure prior to engaging the governance team for new request or renewals," the summary reads.
Surprisingly, coding repository Github - which Microsoft purchased for $7.5bn last year - is also discouraged for use by employees, though it is not banned outright.
Microsoft warned staff not to use the cloud version of Github "for highly confidential types of information, specs or code".
Picking up the Slack
Microsoft's list was published a day after Slack debuted on the New York Stock Exchange with a valuation of over $23bn (£18bn).
The comms vendor opted for the unusual direct listing model, which differs from a traditional IPO in that it allows shareholders to sell stock rather than the company selling shares to raise funds.
"We think a direct listing is a more effective and efficient way to get to a normalised level of supply and demand without the constraints of an IPO," CFO Allen Shim told Reuters.
Last year it raised $427m in Series H funding, which elevated its valuation at the time to $7.1bn.
In an IPO document in April, Slack fingered Microsoft as its primary competitor, while naming Facebook, Google and Cisco as less direct rivals.
Earlier this year, it unveiled apps and integrations to work with Office 365 but didn't work directly with the vendor on the offering, while Microsoft's launch of a free version of Teams was seen as a pot shot at its comms rival.
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