TL;DR
A list could not be expanded this time. Retry with backoff and do not suppress the list address from 4.2.4 alone.
What this code means
X.2.4 describes a mailing-list destination that could not be expanded. The leading 4 makes the observed failure temporary.
Technical meaning
The code names a list-expansion problem, not a single-mailbox quota or a general addressing fault.
Delivery status
Class 4 means the condition may clear and does not prove the list address is permanently invalid.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Use bounded idempotent backoff retries; stop on success, permanent result, or limit exhaustion.
Suppression decision
Do not suppress the list address from 4.2.4 alone; require independent permanent evidence or policy.
Common causes
- A temporary condition blocked expansion of a mailing-list address.
Diagnostic steps
- Confirm the destination was a mailing list, retain the response and attempt time, and ask its administrator to inspect expansion.
Actions by owner
Sender
- Avoid unlimited manual resends and share code and timing with the list contact if retries fail.
Recipient
- Confirm the list address is intended and forward repeated evidence to its administrator.
Recipient administrator
- Inspect list-expansion state and return a permanent result if the condition is confirmed permanent.
Sources
These sources define what this enhanced status code means, mainly through the IANA registry and related RFCs. When provider examples appear on the page, they come from that provider's published documentation. Follow the links to read the original wording in context.
- SMTP Enhanced Status Codes — IANA registry of enhanced mail system status codes.
- RFC 5248 — A Registry for SMTP Enhanced Mail System Status Codes — Creates and governs the IANA enhanced status code registry.
- RFC 2034 — SMTP Service Extension for Returning Enhanced Error Codes — Defines how SMTP returns enhanced status codes to clients.
- RFC 3463 — Enhanced Mail System Status Codes — Defines the class/subject/detail model for enhanced status codes.
Last verified:
