remove email associated with account

I have a zoho mail account with one business. I'd like to set up a separate account and have zoho invoicing for a different business where the email is hosted with Google Apps.

But when I try to set up the new business, zoho says the Google Apps email is already being used. I am guessing that it is in a field somewhere in the first zoho mail setup. How can I remove this relationship?

Thanks,
Helen