With delegated permission MailboxSettings.ReadWrite
you can't read/write outlook categories of other users due to security. Graph Explorer requires the user to log in, and all permissions used are delegated, so you cannot use Explorer.
Only way to read/write outlook categories is with application permission MailboxSettings.ReadWrite
. First you need to create an app in the Entra ID, and then you need to get a token with the client credentials flow, test it in postman.
Hope this helps.
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