SMTP code 450 4.2.1
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.
- Message
- User "is receiving email too quickly" / "at a rate that prevents delivery"
- Meaning and fix
- Rate limit on the recipient's side; resend later
From Gmail SMTP Errors and Troubleshooting · Temporary failures (4xx)
- Message
- "Peak SMTP relay limit exceeded for this customer"
- Meaning and fix
- Workspace SMTP relay limit; contact the account administrator
From Gmail SMTP Errors and Troubleshooting · Temporary failures (4xx)