What does the response code 421 indicate in SMTP?

The response code 421 signifies that the service is not available and that the server is closing the transmission channel. This typically occurs when the server is temporarily unable to handle requests due to high load, maintenance issues, or other temporary unavailability. In such cases, the server proactively informs the client about the temporary situation, prompting the client to cease communication. The message indicates that while the client may have been able to establish a connection, continuing communication is not viable.

Understanding the specific meaning of this code is essential for troubleshooting and effectively managing SMTP communications, especially in scenarios where service interruptions could impact email delivery or user experience. This highlights the importance of interpreting SMTP response codes correctly when diagnosing connectivity issues.