SMTP 421 Service Not Available vs 556 Domain Does Not Accept Mail
SMTP 421 (Service Not Available) is a 4xx Transient Negative response, while 556 (Domain Does Not Accept Mail) 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, 556 means that the destination domain does not accept mail and no forwarding address is available. The domain's DNS configuration (null MX record) explicitly indicates it does not receive email.
Описание
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.
Когда вы это видите
When connecting to a mail server that is overloaded, undergoing maintenance, or shutting down. The server cannot handle your request right now.
Как исправить
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.
Описание
The destination domain does not accept mail and no forwarding address is available. The domain's DNS configuration (null MX record) explicitly indicates it does not receive email.
Когда вы это видите
When sending to a domain that has published a null MX record (RFC 7505) in DNS, explicitly declaring that it does not accept any email messages.
Как исправить
Verify the domain's MX records — a null MX (priority 0, empty host) means the domain intentionally rejects all mail. Contact the recipient through an alternative channel.
Ключевые различия
421 is a 4xx Transient Negative response, while 556 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 556: The destination domain does not accept mail and no forwarding address is available. The domain's DNS configuration (null MX record) explicitly indicates it does not receive email.
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 556 when when sending to a domain that has published a null MX record (RFC 7505) in DNS, explicitly declaring that it does not accept any email messages.
Когда что использовать
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 556 (Domain Does Not Accept Mail): Verify the domain's MX records — a null MX (priority 0, empty host) means the domain intentionally rejects all mail. Contact the recipient through an alternative channel.