new microsoft account alias rejected but unused!

That alias cannot be added if it is already associated with another Microsoft account. Existing @hotmail.com, @live.com, @outlook.com, and @msn.com addresses also cannot be added as aliases to a different Microsoft account, even if they appear unused.

What the reported behavior means:

1. "This email address is already taken" indicates the address is already reserved or associated with a Microsoft account.
2. "We couldn't find a Microsoft account" at sign-in does not override that. It can still fail to be added as an alias if the address is already tied to another Microsoft account state.
3. Microsoft does not support moving, merging, or recycling an Outlook.com address as an alias on another Microsoft account.

What can be done:

1. Go to the Microsoft account how you sign in page: Edit account info
2. Select Add email.
3. Add either:
   • a new Outlook.com email address and attach it as an alias, or
   • an existing non-Microsoft email address that is not already tied to another Microsoft account.

If the goal is to use that exact @outlook.de address on the current account, there is no supported self-service path in the provided guidance when Microsoft reports it as already taken.

References:

• How to add an email address to your Microsoft account
• Change the email address for your Microsoft account
• I can't add another new alias email to my account - Microsoft Q&A Why can't I add a alias email address which is available as like a new account to my personal outlook account as an alias.