Hi @María ,
Currently, the new Outlook does not support the ability to open or save .msg and .eml files, you can switch back to the classic Outlook at any time by clicking the toggle button in the top right corner of the window.
The new Outlook is not yet complete, and some options may not be available or work as expected. Many features of our current products are designed and upgraded based on customers’ feedback. Someone has already submitted this feedback, you can vote for him: Add Save option for emails in "New Outlook" (.msg file)
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