4.2.1

Enhanced Status Code 4.2.1: Mailbox Disabled or Rate Limited - Temporary

Temporary failure Medium severity Mailbox RFC 3463
What it means

Enhanced Status Code 4.2.1 means “Mailbox Disabled or Rate Limited - Temporary.” The recipient mailbox is not accepting messages right now. Gmail uses this specifically when a recipient is receiving mail faster than it will allow, which is a limit on them rather than on you.

At a glance
Code4.2.1
Bounce typeSoft (temporary)
SeverityMedium
CategoryMailbox
What to doQueued and retried automatically
StandardRFC 3463
What it looks like in your mail logs 
450 4.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered

What does 4.2.1 mean?

Enhanced status code 4.2.1 is the temporary form of a mailbox that is disabled or not accepting messages. RFC 3463 registers X.2.1 for a mailbox that is disabled and not accepting messages, and the 4.x form says that state is expected to pass.

Google uses it in a way worth knowing about: 450 4.2.1 appears when a recipient is receiving email too quickly, when the recipient rate prevents additional messages, and when a peak SMTP relay limit has been exceeded. In the first two cases the limit belongs to the recipient, not to you. Someone signed up to many services at once, or a shared address is receiving from many senders, and Google is protecting that mailbox. Your reputation is not implicated and reducing your overall volume will not help.

The third case is different and is worth separating in your reporting. A peak SMTP relay limit is about your relay account rather than the recipient, and it does mean you are sending faster than the relay permits. If you see 4.2.1 concentrated on a small number of recipients it is the first kind; if it appears across many unrelated recipients at once it is the second.

How 4.2.1 plays out

Your server attempts delivery
The recipient defers with a temporary 4.2.1 reply
Your server queues the message and retries on a back-off schedule
It delivers on a later attempt, or becomes a hard bounce if it keeps failing

Where 4.2.1 sits: soft vs hard bounce

Soft bounce (4xx) Hard bounce (5xx)
NatureTemporaryPermanent
SMTP class4xx5xx
What to doLet it retrySuppress the address
Recoverable?OftenNo
4.2.1 is✓ this code

What each provider means by 4.2.1

A registered code does not oblige a provider to use it that way, and the large ones diverge. Match the wording in your own bounce, not the definition above.

Gmail The recipient is receiving mail too quickly, or a peak SMTP relay limit was exceeded Reference

Common causes of 4.2.1

  • The recipient mailbox is temporarily suspended or disabled
  • The recipient is receiving mail faster than the provider allows
  • A shared or role address is being written to by many senders at once
  • Your relay account exceeded a peak sending rate limit
  • The mailbox is being migrated or is briefly unavailable
  • The recipient provider applies a per-sender-to-per-recipient hourly cap

How to fix 4.2.1

  • Distinguish the two cases: concentrated on few recipients means their limit, spread wide means yours
  • Let the queue retry, since recipient rate limits clear within hours
  • Reduce per-recipient frequency if you are sending the same person many messages
  • If a relay limit is named, spread the send over a longer window
  • Do not suppress the address, since this is temporary and the mailbox is usually valid
  • Watch for it becoming 5.2.1, which means the mailbox is permanently disabled

Frequently asked questions

What does 450 4.2.1 mean?
It means the recipient mailbox is not accepting messages at the moment. From Gmail it usually means the recipient is receiving mail faster than Google permits, which is a limit protecting that mailbox rather than a judgement about you. It is temporary, so your server retries and the message normally arrives.
Is 4.2.1 my fault or the recipient's?
Usually the recipient's, and the distribution tells you. If 4.2.1 is concentrated on a handful of addresses, those mailboxes are being rate limited and there is nothing to fix at your end. If it appears across many unrelated recipients simultaneously, look at whether your relay has a peak rate limit you are exceeding.
Should I remove addresses that return 4.2.1?
No. The leading 4 marks it temporary and the mailbox is almost always valid. Suppressing on a soft bounce is one of the most common ways senders damage their own lists. Only consider suppression if it hardens into 5.2.1, which means the mailbox is genuinely disabled.
How is 4.2.1 different from 4.2.2?
They describe different mailbox states. 4.2.2 means the mailbox is full and over quota. 4.2.1 means it is disabled or rate limited, so the storage is not the issue. Both are temporary and both usually resolve without intervention, but only 4.2.2 depends on the recipient clearing space.
What is a peak SMTP relay limit?
It is a cap on how fast a relay account may submit, as opposed to how much it may send in a day. Google applies one to Workspace SMTP relay. Hitting it means your send rate is too high for the account tier rather than that anything is wrong with your mail, and spreading the same volume over a longer window resolves it.

Related bounce codes

5.2.1 Mailbox Disabled or Not Accepting Messages 4.2.2 Mailbox Full - Temporary 450 Mailbox Unavailable - Busy or Temporarily Blocked 5.2.2 Mailbox Full - Over Quota 4.7.28 Mail Flood Detected - Temporary Rate Limit

Sources & further reading

RFC 3463 · The standard that defines this reply code IANA registry · The authoritative list of enhanced status codes and their meanings Gmail SMTP errors and codes · Google Workspace reference for every code Gmail returns
Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← All bounce codes