This error likely occurs when the server refuses to send the message to another server even if you successfully establish the SMTP connection. For instance, this may be due to a lack of authentication for the outgoing mail server or a recipient server misconfiguration.
To solve this issue, you can re-check the authentication and determine there is no issue first. Then, you have to check the headers of the bounced emails and use the details to check whether the emails got rejected because they were considered spamming by the recipient server.
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