When I try to add my non-Google address as an alternate email address to the Google account, Google prevents me from doing this:
I am not aware of any Google account with which my email address is already used, except as a recovery address that I assume is not a reason for this bar (especially considering that Google allows an address to be added as an address). recovery without permission) from the owner of the addresses).
How can I find out which Google account this address is being used with?
Note: This is not answered by any of:
Add Google Apps email to the email addresses in my Google account
How to add an alternate email to the Google account given the error "This address is already used with another Google account."