Please notice: This is how it should work, in theory. However I have not tested this method and post this only as a non-tested idea.
When you sign in to Windows with an MS account and you have set that account up in the Mail app, you cannot delete the account. You can delete all other mail accounts in Mail app but not the first, initial account when signing in with an MS account.
In theory, this workaround should work and achieve what you want to, remove the IMAP account and set it up as a POP3 account:
- Enable POP3 for your Outlook.com / Hotmail.com email account, link in Cliff's post
- If you have set up the Two Step Verification for your Outlook.com / Hotmail.com account you need to set up an app password for that account account at https://account.live.com/proofs/Manage:
- Switch to a local account (tutorial)
- Delete the IMAP account from the Mail app (tutorial, Option Two). When signed in to Windows with a local account you can delete even the initial mail account from Mail app, something not allowed when signed in with an MS account
- Set up your account as POP3 account (tutorial, Option One Step 13). Notice that if you have set up the Two Step Verification you need to use the app password set up in step 2 above instead of your real Outlook.com / Hotmail.com password; your Outlook.com / Hotmail.com password will still be what it is, it remains the same, but Windows Mail app cannot use that password if the Outlook.com / Hotmail.com account is set up as POP3 account and it is using the Two Step Verification. In that case it needs the so called app password
- Switch back to MS account (tutorial)
That's it. From this Microsoft help page you can find the required POP3 and SMTP servers: Set up an email app with Outlook.com - Outlook.com Help