I was wondering the same thing.
For example, I have an SMTP forwarding account with change IP for the domain mydomain.com. So any emails that someone sends to
elam@mydomain.com get redirected through Changeip's DNS servers to my mail server sitting in my apartment. That works great.
So I then purchased a $10.00, 2mb mail account with ChangeIP assuming I could set up a mailbox called
elam@mydomain.com for which I could send outbound mail from my apartment through changeip, which would get forwarded to the recipient, and the forwarded email would show it came from mydomain.com. But when I try to set up the domain name for my mailbox account, it's saying that I need to add a standard domain first but it won't let me add mydomain.com.
So am I to understand that you can't have both SMTP forwarding to/from the same domain?