SMTP 421 Service Not Available vs 554 Transaction Failed
SMTP 421 (Service Not Available) is a 4xx Transient Negative response, while 554 (Transaction Failed) is a 5xx Permanent Negative response. 421 indicates that the SMTP service is not available and the server is closing the transmission channel. This may be sent as a greeting or during a session if the server needs to shut down. In contrast, 554 means that the mail transaction has failed. This is a catch-all permanent error that can occur at any point during the SMTP session, including as a rejection greeting when the server refuses all connections from
Beschreibung
The SMTP service is not available and the server is closing the transmission channel. This may be sent as a greeting or during a session if the server needs to shut down.
Wann Sie es sehen
When connecting to a mail server that is overloaded, undergoing maintenance, or shutting down. The server cannot handle your request right now.
Wie man es behebt
Retry the connection after a delay. If the issue persists, check server status and resources (CPU, memory, disk). Rate limiting or greylisting may also trigger this response.
Beschreibung
The mail transaction has failed. This is a catch-all permanent error that can occur at any point during the SMTP session, including as a rejection greeting when the server refuses all connections from your IP.
Wann Sie es sehen
When the entire mail transaction is rejected — often due to spam filtering, blacklisted IP addresses, malformed messages, or policy violations detected during the DATA phase.
Wie man es behebt
Check your IP against DNS blacklists (DNSBL) and verify your SPF, DKIM, and DMARC records. Review the full error message for specific rejection reasons. If blacklisted, request delisting from the relevant blocklist.
Wesentliche Unterschiede
421 is a 4xx Transient Negative response, while 554 is a 5xx Permanent Negative response.
SMTP 421: The SMTP service is not available and the server is closing the transmission channel. This may be sent as a greeting or during a session if the server needs to shut down.
SMTP 554: The mail transaction has failed. This is a catch-all permanent error that can occur at any point during the SMTP session, including as a rejection greeting when the server refuses all connections from
You encounter 421 when when connecting to a mail server that is overloaded, undergoing maintenance, or shutting down. The server cannot handle your request right now.
You encounter 554 when when the entire mail transaction is rejected — often due to spam filtering, blacklisted IP addresses, malformed messages, or policy violations detected during the DATA phase.
Wann welchen verwenden
For 421 (Service Not Available): Retry the connection after a delay. If the issue persists, check server status and resources (CPU, memory, disk). Rate limiting or greylisting may also trigger this response. For 554 (Transaction Failed): Check your IP against DNS blacklists (DNSBL) and verify your SPF, DKIM, and DMARC records. Review the full error message for specific rejection reasons. If blacklisted, request delisting from the relevant blocklist.