Exact response pattern
550 5.2.4 Mailing list expansion problemPermanentGenericRecipient
What it means
The destination could not expand a list or group address.
Likely causes
- Broken group membership.
- Directory issue.
Recommended response
- Recipient administrator must repair the list.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.