The server that responded to you is one of DH's mail exchange servers. That processes any and all outgoing mail for your cluster. You're not doing anything wrong so far as I can see from that sampling of error message, but it doesn't really tell me anything relevant, save exactly where the email error was encountered. Did it tell you the address in question doesn't exist? Mailbox is full? Couldn't connect to a server it needed to?
Yes, I have an opinion.
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