TL;DR
Temporary recipient rate limit: retry with limits and do not suppress from one 4.2.1.
What this code means
X.2.1 is generic mailbox-not-accepting wording; this concrete class-4 code rests on exact Gmail ReceivingRate evidence, not an IANA class confirmation.
Provider examples
smtp;450 4.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered. Please resend your message at a later time. If the user is able to receive mail at that time, your message will be delivered. For more information, please visit https://support.google.com/mail/?p=ReceivingRate - gsmtp450 4.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered. Please resend your message at a later time. If the user is able to receive mail at that time, your message will be delivered. For more information, please visit https://support.google.com/mail/?p=ReceivingRate - gsmtp<mulka@gmail.com>: host alt1.gmail-smtp-in.l.google.com[124.215.9.62] said: 450-4.2.1 The user you are trying to contact is receiving mail at a rate that 450-4.2.1 prevents additional messages from being delivered. Please resend your 450-4.2.1 message at a later time. If the user is able to receive mail at that 450-4.2.1 time, your message will be delivered. For more information, please 450-4.2.1 visit 450 4.2.1 https://support.google.com/mail/?p=ReceivingRate f15-20020a056402354f00b00548d4febb0bsi965249edd.614 - gsmtp (in reply to RCPT TO command)Technical meaning
The documented Gmail case is a temporary per-recipient receiving-rate limit.
Delivery status
Class 4 may clear; it is not a permanent mailbox result.
- Class
- Temporary failure
- Retry
- Controlled retry
- Suppression
- Check the full context
Retry decision
Use jitter, idempotency, and a retry limit, spacing sends to the recipient.
Suppression decision
Do not suppress on 4.2.1 alone; wait for independent permanent evidence or policy.
Common causes
- An inbound burst, temporary mailbox hold, or protective throttle.
Diagnostic steps
- Retain provider text and determine whether the result is tied to one burst or recurs across senders.
Actions by owner
Sender
- Space retries and avoid unlimited manual resend.
Recipient
- Check unexpected inbound volume or forwarding loops.
Recipient administrator
- Check mailbox throttles, migration, or protective holds.
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.
- Fix bounced or rejected emails — receiving rate limit — Gmail Help article for the recipient receiving-rate limit condition (450 4.2.1).
- smtp-codes (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
- email-bounce-parser (test fixture corpus) — Open-source bounce-parsing library whose test fixtures are pinned locally as evidence.
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