SMTP 450 Mailbox Unavailable vs 500 Syntax Error
SMTP 450 (Mailbox Unavailable) is a 4xx Transient Negative response, while 500 (Syntax Error) is a 5xx Permanent Negative response. 450 indicates that the requested mail action was not taken because the mailbox is temporarily unavailable. This could be due to the mailbox being locked by another process or a temporary policy restriction. In contrast, 500 means 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.
Description
The requested mail action was not taken because the mailbox is temporarily unavailable. This could be due to the mailbox being locked by another process or a temporary policy restriction.
When You See It
When the recipient's mailbox exists but is currently locked, busy, or subject to greylisting. The server is asking you to try again later.
How to Fix
Retry delivery after a few minutes. If using greylisting, the second attempt will typically succeed. For persistent failures, check if the recipient's mailbox is full or disabled.
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.
Key Differences
450 is a 4xx Transient Negative response, while 500 is a 5xx Permanent Negative response.
SMTP 450: The requested mail action was not taken because the mailbox is temporarily unavailable. This could be due to the mailbox being locked by another process or a temporary policy restriction.
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.
You encounter 450 when when the recipient's mailbox exists but is currently locked, busy, or subject to greylisting. The server is asking you to try again later.
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.
When to Use Which
For 450 (Mailbox Unavailable): Retry delivery after a few minutes. If using greylisting, the second attempt will typically succeed. For persistent failures, check if the recipient's mailbox is full or disabled. 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.