SMTP Response Codes

Updated on 2024-12-12 GMT+08:00

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For details about common SMTP response codes, see Table 1.

**Table 1** SMTP response codes
Response Code	Description	Description
214	Help message	Help Information.
220	SMTP Service ready	The SMTP service is ready.
220	Ready to start TLS	Ready to start TLS.
221	Service closing transmission channel	The service closes the transmission channel.
235	Authentication successful	Authentication succeeded.
250	Ok	OK (response upon connection).
334 334	VXNlcm5hbWU6 UGFzc3dvcmQ6	Username encoded using Base64. Password encoded using Base64.
334	Ok	OK (response when PLAIN authentication is used).
354	Start mail input; end with <CRLF>.<CRLF>	Start email input, ending with a dot (.) in a single line.
421	Timeout waiting for data from client	Timeout when waiting for data from the client (disconnection if no message is received within 90 seconds).
500	Syntax error, command unrecognized	Syntax error. The command cannot be identified.
500	The number of sessions exceeds the maximum	The number of sessions exceeds the maximum (10).
501	Syntax error in parameters or arguments	Parameter or syntax error.
501	Email address must be enclosed in angle brackets,like <address>	The email address must be enclosed in angle brackets, for example, <address>.
501	Invalid command argument, not a valid Base64 string	Invalid command parameter. The value is not a valid Base64 string.
502	Command not implemented	The command is not implemented.
503	Refusing any other AUTH command	Any other AUTH command is rejected. (This situation occurs when executing the login command after login.)
530	Invalid email address	Invalid email address.
530	Authentication required	Identity authentication is required.
535	Authentication credentials invalid	Invalid authentication credential.
553	Sender not allowed	Invalid sender. (The sender must be added on the email console.)