EDITED: to correct line breaks.
A couple of points:
1. It rarely takes 72 hours for a new domain to populate.
2. The biggest issue is security. Granted, I tend to be more intense on that topic than some other people, but security needs to come first.
3. Liability. Who's going to pay the legal bills when someone games a deep pocketed domain name owner's domain on a really large MX server, with tens of thousands of users hosted on it?
The long-term solution seems to be using DNS to see where a message should be routed, even when a domain name is built on an originating MX server.
Yes, it means more work, and less automation until the issue is resolved, but it's all about getting the message delivered to the valid location.
While there are probably some easy, short-term solutions, the IETF
will probably have to weigh in on this one to develop a standard in lieu of the generally accepted practice of local delivery first, which is the currently "accept practice."
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