SMTP code 552 5.7.0
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
- "Content presents a potential security issue"
- Meaning and fix
- Blocked attachment or file type (see Gmail's list of blocked file types)
From Gmail SMTP Errors and Troubleshooting · Content, format, and policy