Couldn't create an account because my e-mail is used by somebody else

The email address is already the sign-in name (alias) of an existing Microsoft account, and Microsoft does not support “taking it back” from that account or forcing the other owner to re‑verify it.

Key points from Microsoft account behavior:

1. Aliases are bound to a single Microsoft account
   • Email addresses and phone numbers used to sign in are called aliases and are shared across the same account’s contacts, storage, subscriptions, and settings.
   • An alias cannot be associated with more than one Microsoft account at the same time.
   • “You cannot add an alias that is already associated with another Microsoft account. Aliases cannot be used to share information between two Microsoft accounts.”
2. Cannot move an existing Outlook.com address between accounts
   • Existing @outlook.com, @hotmail.com, @live.com, and @msn.com addresses cannot be added as aliases to another Microsoft account.
   • Even closing the other account does not allow that Outlook.com address to be reused on a different Microsoft account.
3. What can be done on your side
   • Create or use a different email address as the primary alias for a Microsoft account. You can:
     1. Sign in to the Microsoft account website.
     2. Go to Your info → Manage how you sign in to Microsoft.
     3. Use Add email to add a new address (or an address you already own) as an alias.
     4. Make that new alias the primary alias.
   • If the email address in question is actually yours at another provider and is being used on a Microsoft account you do not control, the only remediation path is through Microsoft account recovery and support, but there is no supported way to forcibly detach that alias from the other account.
4. About verification
   • When an email address is added as an alias, Microsoft sends a verification email to that address. Verification is completed only by following the link in that message.
   • If the other account’s owner cannot verify that alias, they may eventually lose the ability to rely on it, but there is no user‑side control to “force” verification or reclaim the alias into a different account.

Given the current Microsoft account system, the email address cannot be reclaimed from another active Microsoft account or forcibly moved to a new one. The only practical option is to use a different email address (or a new Outlook.com address) as the primary alias for a Microsoft account under your control.

References:

• Help with "phone number is already used" in a Microsoft account
• "Something went wrong" with error ADDriverStoreAccessNonLocalException
• Add or remove an email alias in Outlook.com
• How to add an existing outlook.com account to my existing microsoft account? - Microsoft Q&A