SMTP 450 Mailbox Unavailable vs 503 Bad Sequence
SMTP 450 (Mailbox Unavailable) is a 4xx Transient Negative response, while 503 (Bad Sequence) 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, 503 means that the commands were sent in the wrong order. SMTP requires a specific command sequence (EHLO, MAIL FROM, RCPT TO, DATA) and this error indicates a step was skipped or repeated.
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.
Quand vous le voyez
When the recipient's mailbox exists but is currently locked, busy, or subject to greylisting. The server is asking you to try again later.
Comment résoudre
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 commands were sent in the wrong order. SMTP requires a specific command sequence (EHLO, MAIL FROM, RCPT TO, DATA) and this error indicates a step was skipped or repeated.
Quand vous le voyez
When sending DATA before RCPT TO, or MAIL FROM before EHLO. The server enforces the correct SMTP command sequence and you have sent a command out of order.
Comment résoudre
Follow the proper SMTP command sequence: EHLO/HELO first, then MAIL FROM, then RCPT TO (one or more), then DATA. Reset with RSET if you need to start over.
Différences clés
450 is a 4xx Transient Negative response, while 503 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 503: The commands were sent in the wrong order. SMTP requires a specific command sequence (EHLO, MAIL FROM, RCPT TO, DATA) and this error indicates a step was skipped or repeated.
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 503 when when sending DATA before RCPT TO, or MAIL FROM before EHLO. The server enforces the correct SMTP command sequence and you have sent a command out of order.
Quand utiliser lequel
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 503 (Bad Sequence): Follow the proper SMTP command sequence: EHLO/HELO first, then MAIL FROM, then RCPT TO (one or more), then DATA. Reset with RSET if you need to start over.