SMTP error 420 is a transient or "4xx" error code issued by a mail server, signaling a temporary issue causing a delay in processing the email message. Unlike permanent errors denoted by "5xx" codes, SMTP error 420 implies a temporary problem on the server, suggesting that the email delivery might succeed if the sender attempts a retry at a later time.

What's Causing This Error?

SMTP error 420 can result from various temporary issues, including:

1. Server congestion: High incoming email traffic overwhelms the mail server, leading to delays in processing new messages.
2. Greylisting: Some mail servers employ greylisting as a spam-prevention measure. When an unfamiliar sender tries to send an email, the receiving server may initially reject it with a 420 error. If it's a legitimate sender, the sending server will retry delivery after a delay, and the email will be accepted.
3. Temporary server issues: Transient problems such as server hardware or software issues, network problems, or other temporary glitches can trigger SMTP error 420.

How to Fix SMTP Error 420?

1. Wait and retry: Typically, SMTP error 420 is a temporary issue arising from server congestion or other transient problems. The recommended approach is to wait and retry sending the email later, as the receiving server should eventually be able to accept the message.
2. Check for greylisting: If greylisting is suspected as the cause, simply waiting and retrying the email should resolve the issue when the receiving server accepts the email upon the retry.
3. Investigate server issues: If encountering SMTP error 420 consistently when sending emails to a specific recipient, it's possible that the recipient's mail server is undergoing server issues. Contact the recipient's email administrator for assistance.

SMTP Error 420 Examples:

• Example 1: "420 Temporary failure, please try again later."
• Example 2: "420 Greylisted - Try again in 15 minutes."
• Example 3: "420 Service temporarily unavailable due to server maintenance. Please retry later."

In both phpmailer and Jenkins environments, addressing and understanding the nature of SMTP error 420 is crucial for effective email communication.