See my response to your other post.
Even yahoo etc use a primary email address for you to register with, after which they assigne you a new address with your username@yahoo.com.. Same can be acheived with components like Mambomalpro.
With mambomail lite version, you install the module and then any email coming to your domain that matches the username gets forwarded to that members primary email address and not in your site. With the Pro version, you have a complete integrated internet email system that automatically creates the username@mydomain.com email address, accessible by the user on your site and can send, receive email over the net outside of the site. It works..
TO have the email in this system integrate at a deeper level within joomla, you need to hack some code. No way around that.