SMTP 235 Authentication Successful vs 556 Domain Does Not Accept Mail
SMTP 235 (Authentication Successful) is a 2xx Positive Completion response, while 556 (Domain Does Not Accept Mail) is a 5xx Permanent Negative response. 235 indicates that the client has been successfully authenticated using the AUTH command. The server will now accept mail commands from this authenticated session. 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.
Description
The client has been successfully authenticated using the AUTH command. The server will now accept mail commands from this authenticated session.
When You See It
After submitting valid credentials via the AUTH command (LOGIN, PLAIN, or other SASL mechanism). The server has verified your identity and you can now send mail.
How to Fix
No fix needed — authentication succeeded. If you expected this but received a different code, double-check your username, password, and the authentication mechanism.
Description
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.
When You See It
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.
How to Fix
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.
Key Differences
235 is a 2xx Positive Completion response, while 556 is a 5xx Permanent Negative response.
SMTP 235: The client has been successfully authenticated using the AUTH command. The server will now accept mail commands from this authenticated session.
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 235 when after submitting valid credentials via the AUTH command (LOGIN, PLAIN, or other SASL mechanism). The server has verified your identity and you can now send mail.
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.
When to Use Which
For 235 (Authentication Successful): No fix needed — authentication succeeded. If you expected this but received a different code, double-check your username, password, and the authentication mechanism. 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.