TL;DR
The sender system is temporarily failing return-path checks. Check the sender address and return-mail path, retry with limits, and do not suppress the recipient.
What this code means
X.1.8 concerns a sender system that cannot accept return mail. 4.1.8 makes the observed outcome temporary, not a permanent recipient failure.
Provider examples
450 4.1.8 <user@domain.com>: Sender address rejected: Domain not foundTechnical meaning
The domain after @ may be wrong or its receiving system may currently be unable to accept return mail.
Delivery status
Class 4 means the condition may clear; it does not prove that the recipient address is invalid.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Use bounded, idempotent backoff retries and stop on success, a permanent result, or limit exhaustion.
Suppression decision
Do not suppress the recipient based on 4.1.8; the issue concerns the sender-side return-mail path.
Common causes
- The sender domain is misconfigured or the identified sender system cannot accept return mail in this attempt.
Diagnostic steps
- Inspect the raw response and verify the sender domain's intended return-mail handling and logs.
Actions by owner
Sender
- Verify the sender address only against reliable configuration, not by guessing.
Sender administrator
- Repair confirmed sender-address or return-mail configuration and keep retries bounded.
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.
- Common email bounce messages — Rackspace documentation of common SMTP bounce messages.
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