We can't explain it. That's up to the folk that run email ..
That's up to the folk that run the email servers. We can discuss how email servers work but since we don't administer such we can't fix it.
The message is very common when a mail server is overloaded. The sending server will notify the sender of the email that it didn't make it but most servers will try again later. That's why it may go through.
But while all that is interesting to know how it all works, it won't be any one company to contact and even then it's unclear they will be able to do anything (such as Roadrunner?) since most of this is all about the SPAM emails that flood the servers. If you could cut out the spam, we'd likely have more than twice as much email server than we really need.