450 4.1.1 Recipient address rejected: unverified address / mailbox unavailable
The server could not confirm the recipient exists right now and is asking you to try again. "Mailbox unavailable" with a 4 means temporary: the directory lookup failed, the back-end mailbox store was unreachable, or the server verifies recipients against another system that did not answer. Many greylisting setups also use 450 for the first attempt.
What the server replies
450 4.1.1 <dana.fox@example.com>: Recipient address rejected: unverified address: connect to mail.example.com[203.0.113.20]:25: Connection timed out
- Sent by
- Any mail server
- Type
- TemporaryRetry helps
- About
- Recipient problems
- Enhanced code
- 4.1.1
Who sends it
Postfix with reject_unverified_recipient when the probe to the real mailbox server fails; Exchange and other gateways when the recipient directory is unreachable; older Postgrey installs as "450 4.2.0 Recipient address rejected: Greylisted". The 5.1.1 form of the same text is permanent.
Why it happens
- A gateway in front of the real mail server (a spam filter, a relay) could not reach it to verify the address.
- The directory service behind the mail server was down or slow.
- Greylisting: the server rejects the first attempt from an unknown sender and accepts the retry.
- The mailbox is being moved between servers.
How to fix it
Wait for the retry
Most 450 4.1.1 replies resolve on the second or third attempt. Only investigate if the message is still deferred after a few hours.
Check the address anyway
If the deferrals turn into 550 5.1.1 later, the address was bad all along and the 450 was the gateway failing to confirm that.
Admin: check the verification path
On a Postfix gateway using address verification, confirm the transport to the back-end server works and raise address_verify timeouts if the back end is slow.
Check it yourself
These free tools show the records and connections behind this code for your own domain or server.
Related codes
- 550 5.1.1 User unknown / recipient address rejectedThe most common bounce there is: the mailbox does not exist.
- 451 4.7.1 Greylisted, please try again laterThe receiving server has never seen this combination of sending IP, sender address and recipient before, so it refuses the first attempt and remembers the triple.
- 451 4.3.0 Temporary system problem / temporary lookup failureSomething inside the receiving server failed while it was handling your message and it is asking you to retry.
Sources
Fewer bounces to look up.
Faivelo queues temporary failures and retries them on a schedule receivers accept, then tells your application once, in plain words, if a message finally cannot be delivered.
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Other free tools
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- Email address validatorCheck whether an address is well formed, whether its domain accepts mail, and whether it is disposable or a role account.