Error Description
SMTP error 421 is a transient or "4xx" error code returned by a mail server to indicate that it is currently unable to accept incoming email messages. This error is typically used to signal a temporary service unavailability or congestion on the server. SMTP error 421 suggests that the sending server should try again later for successful email delivery.
What's Causing This Error?
SMTP error 421 can 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. If this limit is exceeded, the server may respond with a 421 error.
- Temporary server issues: Server hardware or software issues, network problems, or other transient problems can lead to SMTP error 421.
How to fix SMTP Error 421?
- Wait and retry: SMTP error 421 is usually a temporary issue that can be resolved by waiting and retrying the email delivery later. The receiving server should eventually be able to accept the message.
- Check for rate limits: If you are sending a high volume of emails, make sure you are not exceeding any rate limits imposed by the receiving server. If you are, consider spreading out your email sending over a longer period.
- Investigate server issues: If you consistently encounter SMTP error 421 when sending emails to a specific recipient or domain, it's possible that the recipient's mail server is experiencing server issues. You may want to contact the recipient's email administrator for assistance.
SMTP Error 421 Examples
Example 1: "421 Service temporarily unavailable. Please try again later."
Example 2: "421 Rate limit exceeded for sender@example.com. Try again in an hour."
Example 3: "421 Greylisted - Retry email delivery in 15 minutes"
