In a questionnaire sent to rivals and seen by Reuters, the European Commission is focusing on the period 2016 to 2021. Microsoft introduced Teams in early 2017 to compete with Slack and others in the fast-growing workplace collaboration market.
Slack, bought by business software maker Salesforce.com in July, took its grievance over Microsoft's Teams software to the Commission last year.
Microsoft, which has been handed 2.2 billion euros ($2.6 billion) in EU fines for cases involving so-called tying and other practices in previous decade, declined to comment.
Slack alleged that tying or bundling Teams with Office was illegal and urged the EU competition agency to separate the two.
It said Microsoft pre-installs the workplace chat, it was difficult to un-install and that the U.S. company refused to provide information which would allow rival products to work with Teams and Office.
This has prompted the Commission to ask if bundled products give companies access to data that may increase their market power in both markets and at the same time make it harder for rivals, in particular those with only product, to compete.
It has also asked about barriers to entry or expansion in the workplace apps market, switching costs for customers and the importance of user data protection.
Rivals were asked for a list of customers who have switched to Microsoft Teams or its bundled Office, the percentage of revenue they had lost as a result, as well as the impact of the integrated products on their investments in innovation and the quality and price of their products.
The questionnaire asked if the COVID-19 had boosted demand for workplace apps and how this will evolve post-pandemic.
Other companies which provide workplace apps include Zoom Video Communications, Alphabet's Google, Facebook and Cisco.
($1 = 0.8641 euros)
(Reporting by Foo Yun Chee; Editing by Alexander Smith)
By Foo Yun Chee