What? ... Not offering SMPT setting is like selling a car with no keys.
The whole idea behind custom domain email is that the customer HAS a custom domain; meaning the customer probably HAS a website associated with said custom domain; meaning that the said website WILL need to send emails occasionally (from contact forms, admin related stuff like backups, updates, the list goes on).
Without SMTP properly setup, the website CANNOT do this basic function; meaning the custom domain email that Apple is offering is pointless for a large chunk of customers.
Just saying.