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#11
You seem to be using a Local Account for Windows, and signing into any Microsoft Account separately.
You must have setup the Mail App with some Account it does not do that by itself. When you click on the icon it opens with whatever Mail Account(s) it was setup with.
If you don't want an Account there you delete it from the Account settings (Right click the Account > Account settings > Delete Account).
My Mail App icon opens with 6 accounts of which 2 are MS accounts(Outlook). It was me that set up those with the credentials for each Account.