SMTP 421 Service Not Available vs 553 Mailbox Name Invalid
SMTP 421 (Service Not Available) is a 4xx Transient Negative response, while 553 (Mailbox Name Invalid) 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, 553 means that the requested action was not taken because the mailbox name is syntactically incorrect. The email address does not conform to the expected format.
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 requested action was not taken because the mailbox name is syntactically incorrect. The email address does not conform to the expected format.
Wann Sie es sehen
When the sender or recipient address contains invalid characters, improper formatting, or violates the server's naming conventions.
Wie man es behebt
Check the email address format — ensure it follows the standard [email protected] pattern with valid characters. Remove any spaces, special characters, or encoding issues from the address.
Wesentliche Unterschiede
421 is a 4xx Transient Negative response, while 553 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 553: The requested action was not taken because the mailbox name is syntactically incorrect. The email address does not conform to the expected format.
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 553 when when the sender or recipient address contains invalid characters, improper formatting, or violates the server's naming conventions.
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 553 (Mailbox Name Invalid): Check the email address format — ensure it follows the standard [email protected] pattern with valid characters. Remove any spaces, special characters, or encoding issues from the address.