SMTP Error 431 is a transient or "4xx" error code returned by a mail server, indicating a temporary issue that prevents the server from accepting the email message. This error suggests that the sending server should retry for successful email delivery at a later time.

What's Causing This SMTP Error 431:

SMTP Error 431 may be caused by various temporary issues, including:

• Server Congestion: The mail server is currently handling a high volume of incoming email traffic, leading to delays in processing new messages.
• Rate Limiting: The server may impose limits on the number of emails a sender can send within a specific time frame. Exceeding this limit can result in a 431 error.
• Greylisting: Some mail servers use greylisting as a spam-fighting technique. An initial rejection with a 431 error occurs when an unknown sender attempts to send an email. Legitimate senders will retry the delivery after a delay, and the email will be accepted.
• Temporary Server Issues: Server hardware or software issues, network problems, or other transient problems can lead to SMTP error 431.

How to Resolve SMTP Error 431 - Step-by-Step Solution:

To resolve SMTP Error 431, follow these steps:

• Wait and Retry: This error is typically temporary. Wait and retry the email delivery later, allowing the receiving server time to handle the message.
• Check for Rate Limits: Ensure you are not exceeding any rate limits imposed by the receiving server, especially if sending a high volume of emails. Consider spreading out your email sending over a longer period.
• Investigate Server Issues: If encountering SMTP error 431 consistently, the recipient's mail server may be experiencing server issues. Contact the recipient's email administrator for assistance.

SMTP Error 431 Examples:

• "431 Service temporarily unavailable. Please try again later."
• "431 Rate limit exceeded for sender@example.com. Try again in an hour."
• "431 Greylisted - Retry email delivery in 15 minutes."
• "431 4.3.2 Server experiencing temporary issues. Email delivery delayed."

By addressing the underlying causes and following these steps, you can effectively resolve SMTP Error 431 in both phpmailer and Jenkins environments.

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