This has happened to me a couple of times. My problem/system/bad habit is to buy a .info domain to begin development on a site, because clients are often confused about the status of their domain registration, who the registrar is, etc.
So, upon renaming a domain, I got this help from my hosting company: "/etc/postfix/main.cf was missing a virtual_alias_maps directive. I've
added that, removed that domain from the mydestination line and placed the system hostname in the mydestination line. After doing that and restarting postfix, it's receiving email."
I believe that I've had to add newly created and/or re-named domain names to the main.cf and/or the virtual file manually, as well, but I have been just slogging through the issues, not really noting the fixes, because I don't create THAT many new domains.