I'm trying to add an existing email as an alias but it keep failing to "Email is already taken" eventhough it's my other "accounts.live.com email address"

It seems that you are encountering an issue when trying to add an existing email address as an alias due to the error message stating that the email is already taken. This typically happens if the email address you are trying to add is already associated with another account or if there is a conflict with existing usernames or email addresses in the system.

To resolve this issue, you can try the following steps:

1. Check Existing Accounts: Ensure that the email address you are trying to add is not already being used as a primary or alternate email for another account. You can do this by checking your account settings in Microsoft 365.
2. Use a Different Email: If the email is indeed associated with another account, consider using a different email address that is not currently in use.
3. Delete or Change the Existing Account: If you want to merge accounts, you may need to delete or change the email address of the existing account that is causing the conflict. Make sure to back up any important data before doing this.
4. Contact Support: If you continue to face issues, consider reaching out to Microsoft support for assistance in resolving the account conflict.

Unfortunately, merging accounts directly is not a supported feature in Microsoft 365, so you may need to manage the accounts separately or delete the one you no longer wish to use.

Hello

If the gmail address is currently an alias for another Microsoft account, you need to free it from that account first, before you want to add the gmail to a different Microsoft account.

This is how you do that.

In web browser incognito mode, go tohttps://account.live.com. Sign into the gmail address.

go to Your Info > Sign in preference, Add an alias.

Create new: this will allow you to use the native domain outlook.com to create a new alias.

Once added, make this new alias your primary alias, then you can delete the Gmail as alias, and that gmail is free from this Microsoft account instantly. Then you can add the gmail to the Microsoft account you want.

I do want to point out, by doing so, it doesn't mean you merge two Microsoft accounts. The gmail just becomes the alias/username for one of the Microsoft accounts. Merging Microsoft accounts is not possible.