Email communication is one of the most reliable forms of communication. For personal requirements, there might be various tools available for communication. When it comes to communication between different organisations or businesses, the most formal and reliable way is to send emails. To make your email look authentic and reliable, the email should be sent from your business's domain based email address.
Custom email address has a custom domain attached to it like, firstname.lastname@example.org. To begin with any business or the organization should have a domain. While a website is good to have (in case someone looks up based on your domain), but it is not mandatory to create a custom email address.