421 4.5.1 Too many errors, you have been temporarily blacklisted

This message indicates that the IP address of the server attempting to connect to the SocketLabs SMTP gateway has recently generated a high number of errors on other connections to the gateway over a short period (minutes). This response code is an intentional protective measure against any potential incoming SMTP brute force attacks.

The most common root causes of temporary blacklisting are:

• Invalid SMTP authentication
  • "501 5.7.0 - Authentication required. Please authenticate with CRAM MD5 or AUTH Login."
  • "501 5.7.0 - Authentication failed. User name or password is invalid."
• Syntactically invalid To/From addresses
  • "550 5.1.3 - The destination address was syntactically invalid."
  • "552 5.1.7 - The sender's address was syntactically invalid."

A temporary blacklisting should automatically clear up in a few minutes if you stop generating errors at an excessive rate. You may need to clear your outgoing queues to get your system to stop trying to send the message(s) in question generating an error. Once you have stopped sending messages to the SocketLabs platform, it is recommend you review any SMTP communication logs to try and find the root errors that have caused the blacklisting to occur.

If you have encountered this issue and your mail system does not log errors, or you do not have access to view the logs, please contact our support team for assistance.