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550 5.2.1 [RDNF] Recipient Domain Not Found

The "550 5.2.1 [RDNF] Recipient Domain Not Found" error message indicates that our server was unable to verify the existence of a recipient's email domain - in other words, the part of the email address after the "@" doesn't seem to exist.

It's possible that this problem is temporary, in which case you should try sending your message later. If you get this error repeatedly, you should check with the recipient to verify that their mail server is properly registered in the DNS.

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