In situations where a supplier profile does not have an email address, you will need to enter a placeholder email just to populate the field so that when you either import the supplier file or go to save the supplier profile, the system will check off that the field is populated and save accordingly.
The field is mandatory because the system is designed to allow you to always send transactions (i.e. Purchase Orders) to your suppliers via email.
You can leave a placeholder (i.e. firstname.lastname@example.org) for any required fields, to avoid any errors.