SMTP 450 Mailbox Unavailable vs 521 Host Does Not Accept Mail
SMTP 450 (Mailbox Unavailable) is a 4xx Transient Negative response, while 521 (Host Does Not Accept Mail) 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, 521 means that the host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
설명
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.
이 코드를 보게 되는 경우
When the recipient's mailbox exists but is currently locked, busy, or subject to greylisting. The server is asking you to try again later.
해결 방법
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.
설명
The host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
이 코드를 보게 되는 경우
When connecting to a server that has been intentionally configured to refuse all email, such as a domain that only sends but never receives mail.
해결 방법
Verify you are connecting to the correct mail server by checking the domain's MX records. If the domain truly does not accept mail, contact the recipient through an alternative channel.
주요 차이점
450 is a 4xx Transient Negative response, while 521 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 521: The host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
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 521 when when connecting to a server that has been intentionally configured to refuse all email, such as a domain that only sends but never receives mail.
언제 어떤 것을 사용할지
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 521 (Host Does Not Accept Mail): Verify you are connecting to the correct mail server by checking the domain's MX records. If the domain truly does not accept mail, contact the recipient through an alternative channel.