I have my office 2021 key but not the email address it was registered under.

For a one-time purchase of Office 2021, the product key is permanently tied to the Microsoft account that was used when it was first redeemed. After redemption, activation is done by signing in with that Microsoft account, not by re-entering the key.

To locate or confirm the correct account:

1. Check each possible Microsoft account for an associated Office license:
   • Go to the Microsoft account Services & subscriptions page: https://account.microsoft.com/services
   • Sign in with each personal email address (including older ones) one by one.
   • For the account that has the Office 2021 license, it will appear on that page. That is the account that must be used to activate Office.
2. Try the “forgot account” guidance:
   • Use the steps in Find the account you use with Microsoft 365 to search across email addresses for the one associated with Office.
3. Activate Office on the Mac using the correct account:
   • Open any Office app on the Mac.
   • When prompted to activate, choose Sign in (not “Enter a product key”).
   • Sign in with the Microsoft account that shows the Office 2021 license on the Services & subscriptions page.

If none of the email addresses show an Office license on the Services & subscriptions page, then that product key is associated with a different Microsoft account that cannot be recovered just from the key. In that case, activation with that key is not possible; a new valid license would be required.

References:

• "This product key has already been used" error in Office
• Using product keys with Microsoft 365
• Unlicensed Product and activation errors in Office
• What to try if you can't install or activate Office for Mac
• Activate Office for Mac