Exact response pattern
550 5.2.1 Mailbox disabled, not accepting messagesPermanentGenericMailbox
What it means
The mailbox exists but is disabled from receiving mail.
Likely causes
- Suspended or closed account.
Recommended response
- Suppress the recipient unless the owner confirms reactivation.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.