SMTP error 431 gives a response message that indicates it happened due to insufficient storage or out-of-memory issues. However, this message may be a little deceptive. This error has nothing to do with the receiving/destination server running out of memory or having insufficient storage to complete the delivery.
SMTP error 431 refers to the number of people who have received the email message. It occurs when the receiving server cannot determine which of the recipients it should distribute to.
As mentioned, this issue is caused by too many recipients. So, you can try sending a fresh email message to the failed recipient, making sure they are the only ones who will receive the message.
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