What it looks like in your logs
450 4.2.0 <user@example.com>: Recipient address rejected: Greylisted, please try again in 300 secondsWhat does 450 mean?
RFC 5321 defines 450 as a requested mail action not taken because the mailbox is unavailable, for example busy or temporarily blocked. The key word is temporarily. The receiving server is not saying the address is invalid, only that it will not accept this message right now. Your server should keep the message and try again after a delay. A single 450 is routine, and it tells you nothing bad about your list.
In practice the most common reason is greylisting. A greylisting server rejects the first attempt from a sender, IP and recipient combination it has never seen, on the theory that spam software rarely retries. A legitimate MTA comes back a few minutes later and is accepted. Other triggers include a mailbox that is locked during a migration, a recipient-level rate limit, or a content scanner that is running behind.
Postfix-based servers often answer 450 4.2.0 or 450 4.7.1 with text such as "Greylisted" or "Client host rejected: cannot find your hostname". Gmail uses 450 4.2.1 when a single recipient is receiving mail faster than it can accept. The enhanced code and text matter more than the 450 itself.
Soft or hard bounce?
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| Sender behaviour | Queue and retry with backoff | Stop, suppress the address |
| Reputation risk | Low, unless it repeats for days | High above a 2% bounce rate |
| 450 | ✓ this code |
Common causes
- Greylisting on the receiving server deferred the first delivery attempt from an unknown sender.
- The recipient mailbox is locked for maintenance, migration or a backup.
- The recipient is receiving mail at a rate above the per-mailbox limit.
- A spam or virus scanner on the receiving side is overloaded and asking senders to retry.
- The sending IP has no reverse DNS, and the receiver defers hosts it cannot identify.
How to fix 450
- Make sure your MTA retries 450 replies; most do by default, with a first retry after 5 to 15 minutes.
- Read the enhanced code: 4.2.x points to the mailbox, 4.7.x points to a policy check such as greylisting or rDNS.
- Confirm your sending IP has a matching PTR record with the reverse DNS checker, because many servers defer hosts without one.
- If 450s appear for many recipients at the same domain, check your IP with the blacklist checker and look at your reputation.
- Only treat the address as a bounce if the retry window expires without a successful delivery.
450 questions
What does SMTP error 450 mean?
It means the recipient mailbox is temporarily unavailable, so the server declined this delivery attempt but expects a later one to work. The address is not invalid. Your server keeps the message in its queue and retries. Most 450s disappear on the next attempt, especially when greylisting is the cause.
Can greylisting cause a 450 error?
Yes, greylisting is the single most common source of 450 replies. The server deliberately defers the first message from an unfamiliar sender and accepts the retry a few minutes later. RFC 6647 describes the practice. As long as your server retries from the same IP, the second attempt normally succeeds.
Should I remove an address that returned 450?
No, not after one deferral. A 450 is a soft failure and the address is probably fine. Suppress it only if the same address keeps failing across several sends over multiple days, which suggests an abandoned or broken mailbox. Our bounce rate guide covers sensible soft bounce thresholds.
What is the difference between 450 and 550?
Both refer to an unavailable mailbox, but 450 is temporary and 550 is permanent. A 450 asks you to retry later. A 550 tells you the server has refused the message for good, often because the address does not exist or a policy blocked you.