SMTP Error 421 is a transient or "4xx" error code returned by a mail server, indicating its current inability to accept incoming email messages. This error is commonly used to signal temporary service unavailability or congestion on the server. SMTP Error 421 suggests that the sending server should make subsequent attempts later for successful email delivery.

What's Causing This Error?

SMTP Error 421 can be caused by various temporary issues, including:

1. Server Congestion: The mail server is presently managing a significant influx of incoming email traffic, leading to delays in processing new messages.
2. Rate Limiting: The server may enforce limitations on the number of emails a sender can dispatch within a specific timeframe. Exceeding this limit might trigger a 421 error response.
3. Temporary Server Issues: Transient problems, including server hardware or software glitches, network interruptions, or other temporary issues, can lead to SMTP Error 421.

How to Fix SMTP Error 421?

To resolve SMTP Error 421, follow these steps:

1. Wait and Retry: SMTP Error 421 is typically a temporary issue that can often be resolved by waiting and attempting email delivery later. The receiving server is expected to eventually accept the message.
2. Check for Rate Limits: If you are sending a significant volume of emails, ensure compliance with any rate limits set by the receiving server. If exceeded, consider spacing out your email dispatch over an extended period.
3. Investigate Server Issues: If SMTP Error 421 persists when sending emails to a specific recipient or domain, it might indicate server problems on the recipient's end. Contact the recipient's email administrator for assistance.

SMTP Error 421 Examples

1. "421 Service temporarily unavailable. Please try again later."
2. "421 Rate limit exceeded for sender@example.com. Try again in an hour."
3. "421 Greylisted - Retry email delivery in 15 minutes."

Incorporating these suggested steps can help efficiently address SMTP Error 421 in both phpmailer and Jenkins environments.