If you are seeing the error "421 Message temporarily deferred - [numeric code]", where "[numeric code]" shows a specific diagnostic code (e.g., "4.16.51", "4.16.52") in your SMTP logs, this indicates that:

the message you attempted to send exhibited characteristics indicative of spam,and/or emails from your network have been generating complaints from Yahoo! Mail users. This is a temporary error and your mail server may automatically re-try sending emails at a later time. However, we do encourage you to examine your outbound queues to ensure that spammers are not abusing your mail server. Are you seeing the same error consistently over an extended period of time? If so, we encourage you to provide us with detailed information, including the error and diagnostic code(s) you see in your logs, so that we can help diagnose your problem. For bulk mailers, please visit this page to review our best practice recommendations and request assistance. If your mail server does not primarily send bulk mailings (e.g., you run a personal, corporate, educational, or ISP mail server), please fill out this form. instead. If you’re not the administrator of the mail server in question, please contact the administrator directly with the error message you’re receiving.

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