SMTP code 421 4.7.28
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
- Unusual or unsolicited volume
From Gmail Email Sender Guidelines · Non-compliance error codes
- Message
- "Unusual rate of email" from your IP address / IP netblock / DKIM domain / SPF domain / a URL domain in the body; or "sender exceeded the quota"
- Meaning and fix
- Rate limiting for spam patterns, keyed to that identifier. To recover, wait ≥10 minutes, resume with one connection, and add connections one at a time
From Gmail SMTP Errors and Troubleshooting · Temporary failures (4xx)