Transient (4xx)
450 4.7.1
Greylisting
The receiving server is deliberately, temporarily rejecting the message to see if a legitimate sender retries.
Common causes
- Greylisting: a common anti-spam technique that always rejects the first attempt from an unrecognized sender/IP pair.
What to do
- →Do nothing manually - any standards-compliant SMTP sender retries automatically, and greylisting almost always clears within minutes.
- →If it never clears, check that your retry logic actually re-sends from the same IP the first attempt came from.