I would personally use Mailing lists for leads  and not for customers and  use the emailing/marketing automation functionalities on customers with a specific tag. This way, you can always treat customers in your communication separated from the leads that never have purchased anything.

Now to the technical solution:

with odoo studio, you can create an automation that checks if the tag already exists in the customer and if not adds in after confirmation of a sales order.

With Marketing automation, you create a server action that will update the customer with the tag after there is a sales order for a customer.

Below the automation in the sales order. Should not take more than 10 minutes to do. You will have to create the tag first.

Good Luck!

I'm seconding this request. I watched some Odoo videos but they don't mention this, which seems essential. Can someone please answer this question??