TL;DR
Generic temporary mailbox condition: use bounded retry and do not suppress from this code alone.
What this code means
X.2.0 is the mailbox-family catch-all. This class-4 code is published on exact Comcast 450 evidence, not an IANA-confirmed class assignment.
Provider examples
smtp;450 4.2.0 <example@comcast.net> - Recipient temporarily unavailableTechnical meaning
The reply gives no more specific mailbox condition.
Delivery status
Class 4 may clear and does not establish an invalid recipient.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Retry with backoff, jitter, idempotency, and a limit; stop on success, permanent result, or exhaustion.
Suppression decision
Do not suppress from 4.2.0 alone; require independent permanent evidence or policy.
Common causes
- A provider-side temporary hold or generic recipient deferral.
Diagnostic steps
- Retain recipient, time, and full reply; do not choose a cause from these digits.
Actions by owner
Sender
- Retry with a limit and share the exact reply if it persists.
Recipient
- Check for a provider notice without assuming the code names one.
Recipient administrator
- Check quota, suspension, or policy holds and confirm with the provider.
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.
- RFC 3463 — Enhanced Mail System Status Codes — Defines the class/subject/detail model for enhanced status codes.
- RFC 5248 — A Registry for SMTP Enhanced Mail System Status Codes — Creates and governs the IANA enhanced status code registry.
- SMTP Field Manual (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
Last verified:
