SMTP code 451 4.7.24
What the knowledge base's own tables say about this code, and which articles say it — a description of the reply, never a placement prediction.
The knowledge base documents this code in more than one row. What tells those rows apart is the rejection text in your own log, not the code: read each line below against what your rejection says.
This line comes from a knowledge base article on Gmail, not on the standard.
- Requirement failed
- Suspicious SPF record entries
From Gmail Email Sender Guidelines · Non-compliance error codes
- Message
- "The SPF record of the sending domain has one or more suspicious entries"
- Meaning and fix
- Clean up the SPF record (remove compromised or suspicious includes)
From Gmail SMTP Errors and Troubleshooting · Temporary failures (4xx)