Fastmail is, as its name implies, indeed quite fast. Noticably so compared to others I've tried over the years like aol, Yahoo, Gmail. They allow you to set 'personalities' from the web interface which allow to change the 'from' address. You can even set different 'from' addresses based on what folder the message you're replying from has been moved to, say based on 'rules' you've set. Not free, but fairly priced. Good luck!