SMTP 500 Syntax Error vs 550 Mailbox Not Found
Both SMTP 500 (Syntax Error) and 550 (Mailbox Not Found) 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, 550 means that the requested action was not taken because the mailbox does not exist or the recipient address is rejected by policy. This is a permanent failure indicating the address is invalid or blocked.
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 does not exist or the recipient address is rejected by policy. This is a permanent failure indicating the address is invalid or blocked.
When You See It
When sending to an email address that does not exist on the destination server, or when the server's policy rejects your message (e.g., anti-spam, sender verification failure).
How to Fix
Verify the recipient's email address for typos. If the address is correct, the mailbox may have been deleted or your domain may be blocked — check your sender reputation and SPF/DKIM/DMARC records.
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 550: The requested action was not taken because the mailbox does not exist or the recipient address is rejected by policy. This is a permanent failure indicating the address is invalid or blocked.
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 550 when when sending to an email address that does not exist on the destination server, or when the server's policy rejects your message (e.g., anti-spam, sender verification failure).
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 550 (Mailbox Not Found): Verify the recipient's email address for typos. If the address is correct, the mailbox may have been deleted or your domain may be blocked — check your sender reputation and SPF/DKIM/DMARC records.