451 4.7.1: greylisted or temporarily deferred
The server is deliberately delaying your first message from a new sender.
Temporary failure (soft bounce)
What 451 4.7.1 means
Greylisting rejects the first delivery attempt from an unknown sender with a temporary error, expecting a legitimate mail server to retry. Spammers usually do not, so it filters them out.
Common causes
- First message from your server to this recipient
- A strict anti-spam setup at the recipient
How to fix it
- Make sure your system retries automatically (most do)
- Wait: delivery usually succeeds within 5 to 60 minutes
What to do with the address
Keep the address; this is not a problem with the recipient.
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 451 4.7.1 mean?
- The server is deliberately delaying your first message from a new sender. Greylisting rejects the first delivery attempt from an unknown sender with a temporary error, expecting a legitimate mail server to retry. Spammers usually do not, so it filters them out.
- Is 451 4.7.1 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 451 4.7.1?
- Keep the address; this is not a problem with the recipient.