TL;DR
The recipient mailbox is temporarily unavailable. Verify the address, retry with limits, and do not suppress from one 4.1.1.
What this code means
X.1.1 concerns a bad destination mailbox identifier; 4.1.1 applies the temporary class to the current attempt. The code does not prove a permanent invalid address.
Provider examples
smtp;452 4.1.1 <example@comcast.net> too many recent messages; Try again latersmtp;452 4.1.1 <example@comcast.net> user over quotasmtp;421 4.1.1 <email@example.com>: Recipient address rejected: unverified address: host example.com[x.xx.xx.xx] said: 450 4.1.1 <email@example.com>: Recipient address rejected: unverified address: lost connection with mail.example.com[x.xx.xx.xx] while sending RCPT TO (in reply to RCPT TO command)smtp;421 4.1.1 <email@example.com>: Recipient address rejected: unverified address: Address verification in progressTechnical meaning
The mailbox identifier before @ may be wrong or temporarily unavailable to the receiving system. The complete reply is needed to distinguish those possibilities.
Delivery status
Class 4 permits controlled retry; it is not the permanent X.1.1 outcome.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Retry with backoff, jitter, idempotency, and a limit. Stop on success, a permanent result, or limit exhaustion.
Suppression decision
Do not suppress on 4.1.1 alone; use a confirmed permanent addressing result or policy after reviewing the history.
Common causes
- The local mailbox identifier is wrong, or the receiver cannot find the mailbox during this temporary attempt.
Diagnostic steps
- Compare the recipient address with a trusted value, especially the local part; do not guess a replacement.
Actions by owner
Sender
- Confirm the address with the recipient and allow controlled retry if it remains correct.
Sender administrator
- Retain the response and apply bounded retry before considering the full result history.
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.
- SMTP Field Manual (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
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