Exact response pattern
521 5.2.1 Host does not accept mailPermanentGenericRouting
What it means
The destination host is not configured to receive SMTP mail.
Likely causes
- Wrong MX target.
- Domain intentionally does not accept mail.
Recommended response
- Verify the recipient domain and MX records.
- Do not retry indefinitely.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.