SMTP 450 Mailbox Unavailable vs 550 Mailbox Not Found
SMTP 450 and 550 both indicate a mailbox problem, but they differ in whether the error is temporary or permanent. A 450 is a transient failure (the mailbox exists but is temporarily unavailable), while a 550 is a permanent failure (the mailbox does not exist or the server will never accept mail for it).
説明
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.
このコードが表示される場合
When the recipient's mailbox exists but is currently locked, busy, or subject to greylisting. The server is asking you to try again later.
修正方法
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.
説明
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.
このコードが表示される場合
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).
修正方法
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.
主な違い
450 is transient (4xx) — the sending server should retry delivery later, typically with exponential backoff.
550 is permanent (5xx) — the sending server should not retry and should generate a bounce notification (NDR).
450 often means the mailbox is full, locked, or the server is under greylisting policy.
550 means the email address does not exist, the domain does not accept mail, or the recipient is permanently blocked.
Greylisting uses 450 deliberately to reject first-time senders; legitimate servers retry, spammers do not.
使い分け方
Return 450 when the mailbox temporarily cannot receive mail — it is full, the backend is down, or you are applying greylisting to force a retry. Return 550 when the mailbox definitively does not exist or the recipient is permanently rejected. Getting this right matters: 550 triggers bounce messages, while 450 keeps the message in the sender's queue.