cannot delete old email address. outlook will not allow me to use my existing outlook email address

An existing @outlook.com address cannot be moved onto a different Microsoft account, and two Microsoft accounts cannot be merged into one. If the Outlook.com address already belongs to another Microsoft account, that is why the system says the email is already in use.

What is supported:

1. Keep the Outlook.com account as its own Microsoft account.
2. If the old address is only an alias on the same Microsoft account, go to Manage how you sign in to your account and:
   • add another alias if needed,
   • select Make primary for the address to keep,
   • then select Remove next to the old alias.
3. If the old address is a Microsoft-domain alias such as @outlook.com, @hotmail.com, @live.com, or @msn.com, removing it permanently deletes that alias and it cannot be used again on any Microsoft account.
4. If the address to use is tied to a work or school account, it cannot be made the primary alias for a personal Microsoft account.

If the goal is to stop the old account from appearing in Outlook for Windows, the account can be removed from Outlook:

1. In classic Outlook for Windows, select File.
2. Select Account Settings > Account Settings.
3. Select the account to delete, then select Remove.
4. Confirm the warning. This removes the account from Outlook only; it does not deactivate the email account.

If the sign-in prompt says Which account do you want to use?, that means the same email address exists as both a personal Microsoft account and a work or school account. To reduce that prompt, rename the personal Microsoft account by changing its primary alias, or contact the organization that owns the work or school account.

• Which account do you want to use?
• Add or remove an email alias in Outlook.com
• Change the email address or phone number for your Microsoft account
• How to add an existing outlook.com account to my existing microsoft account? - Microsoft Q&A
• Add an email account to Outlook for Windows