What causes the 421 SMTP error
SMTP code 421 indicates a temporary issue with the server that prevents it from processing requests. This error is not related to the specifics of the email content or the request itself but is generally due to server-side conditions. The server is essentially signaling that it's unable to handle the request at that particular time, but the condition is temporary.
SMTP 421 error explained
Let's break down the SMTP 421 error code into it's parts:
| 4 | The first digit of the SMTP 421 indicates whether the response is good, bad or incomplete. | 4xx Transient Negative Completion. |
|---|---|---|
| 2 | The second digit of the SMTP 421 indicates the response category. | x2x replies generally apply to the transmission channel. |
| 1 | The third digit of the SMTP 421 indicates the specific response. | xx1: While not always true, 0 tends to be a generic response and other numbers are more specific. |
SMTP 421 error code examples
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421 [hostname]: Too much load; please try again later.
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421 [hostname]: Too many concurrent SMTP connections; please try again later.
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421 Too many concurrent SMTP connections; please try again later.
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421 4.7.1 Messages to [recipient domain] deferred due to excessive volume. Try again later - [Link]
Gmail SMTP 421 error examples
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421 4.3.0 Temporary System Problem. Try again later
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421 4.4.5 Server busy, try again later.
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421 4.7.0 IP not in whitelist for RCPT domain, closing connection.
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421 4.7.0 Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily blocked. For more information, visit Prevent mail to Gmail users from being blocked or sent to spam.
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421 4.7.0 Temporary System Problem. Try again later.
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421 4.7.0 TLS required for RCPT domain, closing connection.
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421 4.7.0 Try again later, closing connection. This usually indicates a Denial of Service (DoS) for the SMTP relay at the HELO stage.
What causes the SMTP 421 error?
- Server overload: The server is experiencing a high volume of traffic, exceeding its capacity to handle additional requests.
- Too many concurrent connections: The error is often triggered when a single client establishes too many simultaneous connections to the server.
- Maintenance or updates: Sometimes, servers undergoing maintenance or updates may temporarily reject new connections.
- Network issues: Problems in the server's network infrastructure can also lead to this error.
How do you fix an SMTP 421 error?
In general, 421 errors are temporary. You should wait and try again later. If a time limit is specified, then it's best to wait this amount of time. General best practice is to use an exponential back-off to prevent this error from occurring too frequently, resulting in a block.
You may need to adjust the number of concurrent connections your server makes if this continues to be a problem, or contact the administrator of the domain. If a link is provided, it will generally offer advice.
Need help sending or receiving transactional email? Contact CloudMailin Support to see if we can provide a more reliable SMTP service.
Other SMTP Responses
Troubleshoot other SMTP responses using the links below: