Keyavi Viewer is a browser-based Keyavi application available to external, unlicensed users who need to open Keyavi-protected IKD files. When signing in, you will be prompted to sign in with your existing Google (Gmail), Apple, or Microsoft (Outlook, Live, Hotmail) account.
When determining which email address to use to sign in, consider what email address the sender of the IKD file would have used for you. Remember that they specifically granted you access based on an email address, so use that one to sign in. For example, if the sender typically corresponds with you using your Gmail address, they likely authorized that email address, so you should sign in with your Gmail. Use your regular password associated with that email account.
If you're unsure, you can always contact the sender to confirm.
Currently Keyavi Viewer supports Microsoft accounts, but you can link other accounts, such as Gmail, but following instructions here: How do I sign into Keyavi Viewer