I know that this thread's already been answered, but I have another potential (and easier-to-use) solution for you. Since I have hundreds of domain email addresses, having to pre-create them in mail clients is a royal PiTA. In order to solve that dilemma, I've been using Thunderbird with the
Virtual Identity extension on my Mac for years. The extension automatically detects and replies with the email address that any given email was sent to. It also provides the ability to specify, on-the-fly, custom outgoing From/Reply-To: addresses.