Skip to content
Beta

421 4.7.0: try again later, closing connection

The receiving server is not refusing your mail for good. It closed the connection for a security or reputation reason, and expects your server to try again later.

Updated 3 min read

On this page (7)

421 4.7.0 is a temporary SMTP reply: temporary refusal, connection closed. 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 421 4.7.0 means

421 is "<domain> Service not available, closing transmission channel" [1]. The server ends the session, and the sending server keeps the message and retries later.

X.7.0 is "Other or undefined security status": "Something related to security caused the message to be returned, and the problem cannot be well expressed with any of the other provided detail codes" [2]. The 4 in front makes it temporary.

An SMTP session in which Mailbay's receiving server closes the connection with 421 4.7.0 Too many errors after twenty 550 5.1.1 answers
A real session with Mailbay's receiving server: a sender trying twenty addresses that do not exist gets 550 5.1.1 for each, then 421 4.7.0 and a closed connection.

What Gmail, Outlook and Yahoo say

Gmail
Has eight 421 4.7.0 rows, including "Try again later, closing connection.", "TLS required for RCPT domain, closing connection.", a sending IP without a PTR record, and "This message is suspicious due to the very low reputation of the sending IP address" [3].
Yahoo
"A 421 or 451 SMTP error indicates a temporary problem blocking the delivery of your message", and you "may retry sending at a later time" [4].

Outlook and Microsoft 365 does not list 421 4.7.0 on the error pages we read, so we do not guess what it means 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]Google, Gmail SMTP errors and codes: knowledge.workspace.google.com/admin/support/troubleshooting/gmail-smtp-errors-and-codes
  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 If something here is out of date or wrong, email hello@mailbay.dev and we will fix it.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

  • Low reputation of the sending IP or domain, often a new IP sending too much too soon.
  • No TLS when the receiving domain requires it.
  • A sending IP without a matching PTR record.
  • Too many errors in one session, such as many recipients that do not exist.

How to fix it

  1. Let your server retry. A 421 is temporary, and most mail servers retry on their own for hours or days.
  2. If it keeps happening, check the reason text: TLS, PTR (550 5.7.25) and authentication (550 5.7.26) are quick to fix, and the deliverability test shows authentication and spam scoring for one real message.
  3. Slow down. Spread sends from a new IP or domain over days, and stop sending to addresses that bounce.

How Mailbay reports it

When a recipient's server answers 421 4.7.0 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 and submission servers send 421 4.7.0 in two cases: Too many connections from your address, and Too many errors, closing connection after 20 failed commands in one session, the case in the session below.

Related codes

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

Questions

Do I need to resend after 421 4.7.0?

Usually not by hand. The sending server keeps the message and retries. With Mailbay the message shows as deferred while retries run, and as failed if the mail server gives up.

What is the difference between 421 4.7.0 and 550 5.7.1?

421 4.7.0 is temporary and the connection is closed; the same message may be accepted later. 550 5.7.1 is a permanent refusal of that message.

Why does Gmail say TLS required for RCPT domain?

The receiving domain's administrator requires encrypted connections. Your sending server has to use STARTTLS when it delivers to that domain.

Read next

Join the beta

Mailbay is in private beta and free while the beta runs: inboxes, sending, your own domains and MCP. We invite people in small batches and read every note on the form.

Join the betaRead the docs