As someone else already mentioned, the Fastmail interface makes this super easy. When you respond to an email it by default sends from the email that was addres

As someone else already mentioned, the Fastmail interface makes this super easy. When you respond to an email it by default sends from the email that was addressed. And when you send emails you can enter anything in the from field for your own domains. The only issue I have had is non-technical people getting confused when I have their business name in my email address.