450 SMTP bounce: mailbox unavailable
What 450 means
The recipient mailbox is temporarily unavailable, for example because it is busy or a policy temporarily blocks it. The sender should retry.
Message text
Requested mail action not taken: mailbox unavailable (e.g., mailbox busy or temporarily blocked for policy reasons)
Common causes of 450
- Recipient mailbox is busy or temporarily locked
- Temporary policy block on the receiving side, such as a rate limit
How to fix 450
- Retry later; the sending queue does this automatically.
- If the same address keeps returning 450 for days, confirm the address with the recipient through another channel.
- If you send in bursts, spread volume out and check your IP with blocklist check.
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How to read 450
The first part, 450, is the SMTP reply code. Requested mail action not taken, mailbox unavailable: the mailbox is busy or temporarily blocked. Replies that start with 4 are temporary and replies that start with 5 are permanent (RFC 5321 §4.2.1).
This reply carries no enhanced status code. The three-digit reply code is the only machine-readable signal, so read the text that follows it to learn the reason.
Sending servers should retry a deferred message. RFC 5321 §4.5.4.1 advises waiting at least 30 minutes between attempts and trying for at least 4 to 5 days before giving up and returning a bounce. If the deferral keeps repeating, fix the cause rather than waiting.
Official source: www.rfc-editor.org. Last verified 2026-10-20.
Related guides
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
- How to set up a DMARC record, from p=none to p=reject
Related codes
- 421 SMTP: service not available
- 451 SMTP: local processing error
- 452 SMTP: insufficient storage
- 4.2.2 SMTP: mailbox full
- 4.4.1 SMTP: no answer from host
- 4.7.0 SMTP: security status
FAQ
What does 450 mean?
The recipient mailbox is temporarily unavailable, for example because it is busy or a policy temporarily blocks it. The sender should retry. This is a temporary failure.
How do I fix 450?
Retry later; the sending queue does this automatically. If the same address keeps returning 450 for days, confirm the address with the recipient through another channel. If you send in bursts, spread volume out and check your IP with blocklist check.
Is 450 permanent, and should I retry?
450 is a temporary failure. Your mail server normally retries by itself; if it keeps repeating, fix the cause.