Depends on your setup I suppose. I use FastMail for this purpose and it both automatically sets the `From` based on the `To` of the email I'm responding to and

Depends on your setup I suppose. I use FastMail for this purpose and it both automatically sets the `From` based on the `To` of the email I'm responding to and allows me to click in to the `From` email and type in whatever I want for the local part before sending.