You cannot delete an email address that is marked as primary. If you use any/all of the following Zoho Finance applications, go to each app and check the Sender Email preferences.
If the email address you are trying to delete is marked as the primary email in any of them, you will be prohibited from deleting the email address in Zoho Books. Instead, you can mark an alternate email address as primary in all of the apps and then try deleting.
Books
Online accounting software
for small businesses.