Exact response pattern
550 5.1.3 Bad destination mailbox address syntaxPermanentGenericRecipient
What it means
The recipient address syntax is invalid.
Likely causes
- Malformed local part.
- Invalid characters or formatting.
Recommended response
- Normalize and validate the address before sending.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.