SMTP 500 Syntax Error vs 553 Mailbox Name Invalid
Both SMTP 500 (Syntax Error) and 553 (Mailbox Name Invalid) belong to the 5xx Permanent Negative category. 500 indicates that the server could not recognize the command due to a syntax error. The command line was malformed or the command is not recognized by this server. Meanwhile, 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.
Description
The server could not recognize the command due to a syntax error. The command line was malformed or the command is not recognized by this server.
When You See It
When you send a command the server does not understand — often due to a typo, unsupported command, or an excessively long line.
How to Fix
Check the command spelling and syntax against the SMTP specification. Ensure the command line does not exceed 512 characters and uses proper CRLF line endings.
Description
The requested action was not taken because the mailbox name is syntactically incorrect. The email address does not conform to the expected format.
When You See It
When the sender or recipient address contains invalid characters, improper formatting, or violates the server's naming conventions.
How to Fix
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.
Key Differences
SMTP 500: The server could not recognize the command due to a syntax error. The command line was malformed or the command is not recognized by this server.
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 500 when when you send a command the server does not understand — often due to a typo, unsupported command, or an excessively long line.
You encounter 553 when when the sender or recipient address contains invalid characters, improper formatting, or violates the server's naming conventions.
When to Use Which
For 500 (Syntax Error): Check the command spelling and syntax against the SMTP specification. Ensure the command line does not exceed 512 characters and uses proper CRLF line endings. 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.