The primaru primary intent of the Amahi Mail System is to allow you to pull all ISP email into your server. Keep in mind this is still under development.
It allows you to send/receive mail from your HDA. There are some quirks depending on if you have a registered domain with a paid service provider (i.e. [http://www.godaddy.com GoDaddy], [http://www.hostgator.com Host Gator]). With these services, there is typically what is called a MX record associated with the domain name. This will allow you to send/receive mail as your hostname (i.e. myhda@home.com).
If you use the free ones (i.e. user.yourhda.com, [http://www.no-ip.org no-ip.org]), you will have to masquerade your email address as your ISP (i.e. myhda@yourisp.com). This will ensure you can receive email replies from those you send from your HDA.