SMTP code 553 5.1.7
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.
This line comes from a knowledge base article on Gmail, not on the standard.
- Message
- "The sender address is not a valid RFC 5321 address"
- Meaning and fix
- Fix the syntax of the envelope sender
From Gmail SMTP Errors and Troubleshooting · Recipient and addressing