Sign up for zoho mail without having an email
Currently, people can't sign up for zoho mail (same as registering a zoho account) if they don't have an email address.
Make a registration page for zoho mail different from the registration page of the rest of zoho services (in other words, zoho account registration page). The only difference may be that "zoho mail registration page" won't ask for email, because obviously the associated email address to the zoho account will be the one that is being created.
If not, how can I recommend zoho mail to people? "hi, sign up for a zoho mail account! but first go and create an email somewhere else so you can register in zoho mail".
That is my suggestion.
Regards.