4.x.x transient failure
SMTP Error Code 450: Meaning, Causes and How to Fix It
SMTP 450 is a temporary (transient) failure.
The requested action did not happen because the mailbox was briefly unavailable, for example locked, busy, or lightly rate limited. The sending server keeps the message queued and retries it.
- Class
- 4xx transient
- Bounce type
- Soft bounce
- Highest severity
- Medium
- Responses on record
- 10
Barracuda
Email Security.cloud
Google
ImprovMX
Infomaniak
Outlook
Yahoo Inc
YandexSMTP 450 responses from email providers
How mailbox providers phrase a 450. Open a provider for the full text, severity and fix for every variation.
Comcast1 response
Google1 response
ImprovMX1 response
Infomaniak1 response
Outlook1 response
Yahoo Inc1 response
Yandex1 response
SMTP 450 responses from spam filters
How spam-filtering gateways phrase a 450 rejection.
Barracuda1 response
Email Security.cloud1 response
SMTP 450 questions
›Is SMTP 450 a hard bounce or a soft bounce?
It is a transient failure, so a soft bounce. The sending server queues the message and retries it automatically for a while (often 24 to 72 hours) before it gives up.
›What causes an SMTP 450 error?
The requested action did not happen because the mailbox was briefly unavailable, for example locked, busy, or lightly rate limited. The sending server keeps the message queued and retries it. The exact reason varies by receiver, which is why this page lists the specific 450 responses recorded for Barracuda, Comcast, Email Security.cloud, Google, ImprovMX, Infomaniak.
›How do I fix an SMTP 450 bounce?
Read the enhanced status code and the message text, not just the number. Wording that names the recipient (user unknown, mailbox unavailable) means correct or remove the address. Wording that names authentication or reputation (SPF, DKIM, DMARC, blocklist) means fix that on the sending side, or clear the listing. Every response on this page has its own guidance.