Exact response pattern
550 5.1.2 Bad destination system addressPermanentGenericDNS
What it means
The destination domain or system address is invalid.
Likely causes
- Misspelled domain.
- Domain does not exist.
Recommended response
- Correct the domain.
- Check DNS and MX records.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.