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.
Each digit in an SMTP response code carries a specific meaning.
Transient Negative
A temporary condition prevented completion. The sender should retry.
Connection
Relates to the transmission channel.
Specific Condition
The third digit narrows the response. Zero is typically generic; higher values are increasingly specific.
Typical server responses you may encounter.
421 [hostname]: Too much load; please try again later.
421 [hostname]: Too many concurrent SMTP connections; please try again later.
421 Too many concurrent SMTP connections; please try again later.
421 4.7.1 Messages to [recipient domain] deferred due to excessive volume. Try again later - [Link]
421 4.3.0 Temporary System Problem. Try again later
421 4.4.5 Server busy, try again later.
421 4.7.0 IP not in whitelist for RCPT domain, closing connection.
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.
421 4.7.0 Temporary System Problem. Try again later.
421 4.7.0 TLS required for RCPT domain, closing connection.
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.
Why you might receive this response.
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 with email delivery?
CloudMailin can help troubleshoot and resolve SMTP issues.
SMTP code 421 indicates a temporary issue with the server that prevents it from processing requests
SMTP code 450 is a generic temporary error and is used in a wide variety of conditions by the receiving server
SMTP code 451 indicates a temporary local error on the receiving server
SMTP code 452 is used to indicate that the recipient's email server is unable to accept the message due to a lack of storage space, too many recipients, or insufficient system resources
SMTP code 454 indicates an authentication issue, typically suggesting that the server is denying relay access due to a lack of proper authentication from the sender
SMTP code 455 is used to indicate a temporary issue with the server, where it suggests retrying the request at a later time
SMTP code 530 typically indicates that authentication is required to proceed with the requested email operation
SMTP code 550 is a permanent failure notice, typically indicating that the recipient's email address is invalid or unavailable
SMTP code 552 generally indicates a failure due to storage issues or a limit imposed by the recipient's mail server
SMTP code 553 is used to indicate a rejection due to the recipient's email address being invalid, typically in scenarios where the address is not recognized by the recipient's mail server
SMTP code 554 is a permanent failure notice, indicating that the message has been rejected and will not be delivered