Exact response pattern
552 5.3.4 Message size exceeds fixed maximum message sizePermanentGenericContent
What it means
The message exceeds the receiver maximum size.
Likely causes
- Large attachment or inline media.
Recommended response
- Reduce the message size.
- Check size after MIME encoding.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.