SMTP 354 Start Mail Input vs 535 Authentication Failed
SMTP 354 (Start Mail Input) is a 3xx Positive Intermediate response, while 535 (Authentication Failed) is a 5xx Permanent Negative response. 354 indicates that the server is ready to receive the message body. The client should begin sending the email content (headers and body) and terminate with a single line containing only a period (CRLF.CRLF). In contrast, 535 means that the authentication credentials provided are invalid. The username, password, or authentication mechanism was rejected by the server.
Description
The server is ready to receive the message body. The client should begin sending the email content (headers and body) and terminate with a single line containing only a period (CRLF.CRLF).
When You See It
After sending the DATA command. The server is now waiting for you to type or stream the email message, ending with a dot on its own line.
How to Fix
Send your message content followed by a line with just a period (.) to signal the end. If you get an error after this, check that your message does not contain a bare period on a line by itself within the body.
Description
The authentication credentials provided are invalid. The username, password, or authentication mechanism was rejected by the server.
When You See It
After submitting incorrect credentials via the AUTH command. The username or password does not match any account on the server.
How to Fix
Double-check your username and password. If using an app password (e.g., Gmail, Outlook), ensure it is current and has not been revoked. Also verify the correct SASL mechanism (PLAIN, LOGIN, etc.).
Key Differences
354 is a 3xx Positive Intermediate response, while 535 is a 5xx Permanent Negative response.
SMTP 354: The server is ready to receive the message body. The client should begin sending the email content (headers and body) and terminate with a single line containing only a period (CRLF.CRLF).
SMTP 535: The authentication credentials provided are invalid. The username, password, or authentication mechanism was rejected by the server.
You encounter 354 when after sending the DATA command. The server is now waiting for you to type or stream the email message, ending with a dot on its own line.
You encounter 535 when after submitting incorrect credentials via the AUTH command. The username or password does not match any account on the server.
When to Use Which
For 354 (Start Mail Input): Send your message content followed by a line with just a period (.) to signal the end. If you get an error after this, check that your message does not contain a bare period on a line by itself within the body. For 535 (Authentication Failed): Double-check your username and password. If using an app password (e.g., Gmail, Outlook), ensure it is current and has not been revoked. Also verify the correct SASL mechanism (PLAIN, LOGIN, etc.).