The new Outlook contains a design flaw that does not allow you to drag and drop attachments and documents directly into online storage such as OneDrive, SharePoint and other cloud services. SingleCase is also affected by this issue, which can complicate things for users who are used to easily moving documents between applications.
This limitation is part of the beta version of New Outlook, in which Microsoft is still making adjustments and gathering feedback from users. If you miss this feature, we recommend providing feedback directly to Microsoft so that this issue can be addressed in future updates. More information can be found directly on Microsoft's website, for example here: https://answers.microsoft.com/en-us/outlook_com/forum/all/cant-drag-and-drop-from-new-outlook-desktop-app-to/49b3fea0-1f3a-4349-86b6-c3363281462a
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