Where did you get that idea? I said, “Dynamic IP ranges are pretty much universally considered a spammy trait. You can’t run a reliable mail server from a dynamic IP.” I didn’t say anything about a domain. I’m saying you cannot send mail reliably from a dynamic IP–you need to send it through your ISPs SMTP server (if they offer on), or through a server on a static IP.
The "what domain does the mail come from" and "how do I register and setup a domain" questions are completely unrelated to sending mail from a server on a dynamic IP. 
What I’m saying is: Find out how to send mail through your ISP, and setup the “relayhost” directive in Postfix on your server. Do not try to send mail directly from your server to the world at large. It will either fail to work entirely (if your ISP filters port 25) or it will work very unreliably (because a dynamic IP is extremely spammy).