SMTP 500 Syntax Error vs 551 User Not Local
Both SMTP 500 (Syntax Error) and 551 (User Not Local) 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, 551 means that the recipient is not local to this server and the server does not accept mail for forwarding. Unlike 251, this is a rejection — the server will not relay the message.
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 recipient is not local to this server and the server does not accept mail for forwarding. Unlike 251, this is a rejection — the server will not relay the message.
When You See It
When sending to a recipient that has moved or whose mailbox is hosted elsewhere, and this server refuses to forward the message to the new location.
How to Fix
Check the server's response for a forwarding address and resend directly to that address. Update your contact records if the recipient has permanently moved to a new mail server.
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 551: The recipient is not local to this server and the server does not accept mail for forwarding. Unlike 251, this is a rejection — the server will not relay the message.
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 551 when when sending to a recipient that has moved or whose mailbox is hosted elsewhere, and this server refuses to forward the message to the new location.
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 551 (User Not Local): Check the server's response for a forwarding address and resend directly to that address. Update your contact records if the recipient has permanently moved to a new mail server.