450
SMTP Error 450: Mailbox Unavailable - Busy or Temporarily Blocked
Temporary failure
Medium severity
Mailbox
RFC 5321
What it means
SMTP Error 450 means “Mailbox Unavailable - Busy or Temporarily Blocked.” The recipient mailbox is temporarily unavailable. This soft bounce can be caused by greylisting, temporary mailbox locks, server-side processing, or anti-spam measures that require senders to retry.
At a glance
Code450
Bounce typeSoft (temporary)
SeverityMedium
CategoryMailbox
What to doQueued and retried automatically
StandardRFC 5321
Jun 10 09:14:52 mx postfix/smtpd[20413]: NOQUEUE: reject: RCPT from mail.example.net[203.0.113.10]: 450 4.2.0 <rcpt@example.com>: Recipient address rejected: Greylisted, please try again in 300 seconds; from=<sender@example.net> proto=ESMTPWhat does 450 mean?
SMTP code 450 indicates the requested mailbox is temporarily unavailable. Unlike the permanent 550 error, a 450 means the delivery might succeed if retried later. This code is commonly used by greylisting systems, which deliberately reject first-time sender/recipient pairs to filter out spam bots that typically do not retry.
Many anti-spam systems use 450 as a deliberate filtering mechanism. Postfix greylisting, for example, returns 450 on the first delivery attempt and accepts the email on the second attempt (usually 5-15 minutes later). If you are seeing 450 bounces consistently, ensure your MTA is configured to retry. Single occurrences of 450 are completely normal and expected.
How 450 plays out
Your server attempts delivery
The recipient defers with a temporary
450 replyYour server queues the message and retries on a back-off schedule
It delivers on a later attempt, or becomes a hard bounce if it keeps failing
Where 450 sits: soft vs hard bounce
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| SMTP class | 4xx | 5xx |
| What to do | Let it retry | Suppress the address |
| Recoverable? | Often | No |
| 450 is | ✓ this code |
Common causes of 450
- Greylisting - server deliberately rejects first delivery attempt
- Mailbox temporarily locked for maintenance or migration
- Anti-spam system requesting retry to verify legitimate sender
- Server-side processing delay (virus scanning, content filtering)
- Rate limiting applied to the specific recipient or domain
How to fix 450
- Ensure your MTA has proper retry logic (most handle this automatically)
- Wait 5-15 minutes for greylisting, then retry
- If persistent across many recipients at the same domain, check your sender reputation
- Verify your sending IP is not blacklisted using a Blacklist Checker
Frequently asked questions
What does SMTP error 450 mean?
SMTP error 450 means "Requested mail action not taken, mailbox unavailable," indicating a temporary failure in delivering your email. This soft bounce occurs when the recipient's mailbox is temporarily inaccessible due to being locked, over its storage quota, undergoing maintenance, or when the receiving server is using greylisting as a spam prevention measure. Unlike the permanent 550 error, a 450 error signals that delivery may succeed if retried later.
What is the difference between SMTP error 450 and 550?
The critical difference is that 450 is a temporary failure while 550 is a permanent failure. With a 450 error, your mail server will automatically retry delivery over a period of hours or days, and the email will likely be delivered once the temporary condition clears. With a 550 error, the server immediately bounces the message with no retry. A 450-triggered bounce notification may take 3-5 days to reach the sender (after all retries are exhausted), whereas a 550 bounce is returned immediately.
How do I fix SMTP error 450 "mailbox unavailable"?
In most cases, SMTP error 450 resolves itself without any action because your mail server will automatically retry delivery. Wait 15-30 minutes and check whether the email was eventually delivered. If the error persists, verify the recipient's email address is correct and has no typos, check whether your sending IP has been temporarily rate-limited or greylisted, and ensure your domain's DNS and MX records are properly configured. If you consistently receive 450 errors to the same recipient, contact them through an alternative channel to confirm their mailbox is active.
Can greylisting cause a 450 error?
Yes, greylisting is one of the most common causes of a 450 error. Greylisting servers intentionally return a 450 (or 451) temporary rejection to unknown senders on the first delivery attempt, then accept the message when the sender retries after a short delay. This technique filters out spam bots that do not retry, while legitimate mail servers automatically queue and reattempt delivery within minutes. The greylisting delay is typically 1-15 minutes, after which your sender/recipient combination is whitelisted for future deliveries.
Does a 450 error mean I should remove the email address from my list?
No, you should not immediately remove an address that returns a 450 error, because it is a temporary condition that may resolve on its own. Unlike a 550 hard bounce (which warrants immediate removal), a 450 soft bounce indicates the mailbox may become available again. However, if the same address consistently returns 450 errors over multiple days or across several sending attempts, it may indicate a persistently full or abandoned mailbox, and you should consider removing or suppressing it to protect your sender reputation.
Related bounce codes
Sources & further reading
Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026
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