I do it but you need a client that supports it in replies. The Fastmail web UI allows you to send from the custom email address. Clients like iOS Mail demand

I do it but you need a client that supports it in replies. The Fastmail web UI allows you to send from the custom email address. Clients like iOS Mail demand you configure an account-per-sender rather than letting you add an arbitrary address to the from: line. I end up in a situation where if I don't pay close attention I leak my "main" email address.