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451 4.7.1: try again later

The receiving server is asking your server to come back later. Rate limits and greylisting are the usual reasons, and a normal mail server handles it without you.

Updated 2 min read

On this page (7)

451 4.7.1 is a temporary SMTP reply: deferred: rate limit or policy. This page is part of our SMTP error code reference, where every code links to the RFC and to what Gmail, Outlook and Yahoo publish about it.

Kind
Temporary: the sending server keeps the message and retries
Retry by hand
No, retries are automatic
In Mailbay
deferred, then failed if retries run out

What 451 4.7.1 means

451 is "Requested action aborted: local error in processing" [1]. X.7.1 is "Delivery not authorized, message refused" [2], and the 4 makes it temporary: not now, maybe later. The IANA registry lists 451 among the basic codes used with X.7.1 [3].

An SMTP session in which Mailbay's receiving server answers MAIL FROM with 451 4.7.1 Too many messages from your address after 100 transactions
A real session with Mailbay's receiving server: after 100 MAIL FROM commands from one IP address in a few seconds, the next one gets 451 4.7.1. The 100 accepted ones are left out of the image.

What Gmail, Outlook and Yahoo say

Yahoo
"A 421 or 451 SMTP error indicates a temporary problem blocking the delivery of your message"; "you may retry sending at a later time" [4].

Gmail and Outlook and Microsoft 365 do not list 451 4.7.1 on the error pages we read, so we do not guess what they mean by it.

Sources, read on October 14, 2026

  1. [1]RFC 5321, Simple Mail Transfer Protocol, section 4.2: www.rfc-editor.org/rfc/rfc5321#section-4.2
  2. [2]RFC 3463, Enhanced Mail System Status Codes: www.rfc-editor.org/rfc/rfc3463
  3. [3]IANA, SMTP Enhanced Status Codes registry: www.iana.org/assignments/smtp-enhanced-status-codes/smtp-enhanced-status-codes.xhtml
  4. [4]Yahoo Sender Hub, SMTP error codes: senders.yahooinc.com/smtp-error-codes/

Prices change. If something here is out of date or wrong, email hello@mailbay.dev and we will fix it.

Common causes

  • A rate limit: too many messages or connections from one IP or sender in a short time.
  • Greylisting: some servers refuse the first attempt from an unknown sender and accept a retry.
  • A new sending IP or domain without reputation yet.

How to fix it

  1. Let the sending server retry. Do not resend by hand, which only adds to the rate.
  2. If it lasts for hours, lower your sending rate and check authentication (550 5.7.26).

How Mailbay reports it

When a recipient's server answers 451 4.7.1 to mail you send through Mailbay, our mail server keeps the message and retries on its own. The recipient shows as deferred, and you get a message.deferred event with the server's reply in response. If the retries run out, the message becomes failed with a message.failed event. The message statuses page lists every state.

Mailbay's receiving server answers 451 4.7.1 Too many messages from your address, try again later after 100 messages from one IP address in 10 minutes, and Too much mail for this address when one workspace's addresses get more than 300 recipients in 10 minutes. The submission server answers 451 4.7.1 when one client address or one API key sends too fast, and when a workspace in its first 7 days has sent to 100 recipients that day, which the API reports as daily_send_limit_reached (plan limits).

A Mailbay API response with error code daily_send_limit_reached, its message and the fix, for a workspace in its first week
The same limit through Mailbay's API: a workspace in its first 7 days that has used its 100 recipients for the day gets daily_send_limit_reached. Over SMTP, Mailbay answers this as 451 4.7.1.

Related codes

  • 421 4.7.0: temporary refusal, connection closed, temporary.
  • 550 5.7.1: refused for a policy reason, permanent.
  • 550 5.7.26: failed SPF, DKIM or DMARC, permanent.
  • 535 5.7.8: login rejected, permanent.
  • 552 5.3.4: message too big, permanent.

Seeing an API error rather than an SMTP code? Mailbay's own errors, each with a fix, are on the errors page.

Questions

Is 451 4.7.1 a bounce?

No. It is a deferral. The message stays in the sending server's queue and is retried. With Mailbay it shows as deferred, and only becomes failed if the mail server gives up.

How long should I wait after 451 4.7.1?

Let your mail server's retry schedule handle it. If the same receiver keeps answering 451 for hours, slow down and check authentication.

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