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This is an SMTP error response from the destination system. Most likely it means that the host server is not identifying itself with a fully qualified domain name (FQDN) to the destination. I'm not sure there's much you could do to fix that besides ask your hosting support. Other options are that the admin's email service does not allow relaying or the SMTP authentication encryption settings are not quite correct.
If you can't make headway there try switching to sendmail() instead of SMTP.
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