450 4.2.1: mailbox temporarily unavailable
The mailbox cannot take mail right now, often because it is receiving too much.
Temporary failure (soft bounce)
What 450 4.2.1 means
A temporary refusal. Gmail, for example, uses 4.2.1 when the recipient is receiving mail at a rate that prevents more being delivered. The server expects you to retry later.
Common causes
- The recipient's mailbox is receiving a burst of mail
- The server is temporarily overloaded
- Greylisting on some systems
How to fix it
- Retry after a few hours
- Avoid sending several messages to the same recipient at once
What to do with the address
Keep the address; retry. Only remove it if it fails over many sends.
Check an address now with the free email checker, or look at a domain's mail setup with the email domain checker, MX lookup and SPF checker.
Related error codes
Frequently asked questions
- What does 450 4.2.1 mean?
- The mailbox cannot take mail right now, often because it is receiving too much. A temporary refusal. Gmail, for example, uses 4.2.1 when the recipient is receiving mail at a rate that prevents more being delivered. The server expects you to retry later.
- Is 450 4.2.1 a hard bounce or a soft bounce?
- Temporary failure (soft bounce). It may succeed on a later attempt.
- What should I do with the address after 450 4.2.1?
- Keep the address; retry. Only remove it if it fails over many sends.