Exact response pattern
550 5.1.5 Destination mailbox address validInfoGenericProtocol
What it means
This enhanced code may appear in address verification contexts and indicates a valid destination, despite unusual pairing with a basic code in some implementations.
Likely causes
- Nonstandard server behavior.
Recommended response
- Interpret the full SMTP transcript, not the enhanced code alone.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.