Should I retry a 421 4.7.0 error?

Yes, and your sending server does it automatically. 421 is a temporary deferral, and legitimate mail usually arrives on a retry within minutes or hours. Only investigate if it persists across many retries.

How do I fix SMTP 421 4.7.0?

Let the retry happen. A properly configured sending server retries automatically over several hours.

Is 421 4.7.0 a hard bounce or a soft bounce?

421 4.7.0 is a soft bounce. It is temporary, and your sending server retries it automatically. Your sending server retries on its own. Investigate only if it keeps failing.

Will verifying my list prevent 421 4.7.0?

Only partly. Temporary deferrals are not about the address, but providers defer senders whose mail looks risky, and a high rate of hard bounces is one of the signals they use.

Temporary — will retry

Soft bounceTemporary; the server retries

SMTP 421 4.7.0

temporarily deferred, try again later

SMTP 421 4.7.0 is a temporary deferral: the recipient's server is refusing the message for now and asking you to retry. It is the most common code behind greylisting and rate limiting, and legitimate mail usually arrives on the retry minutes later without any action from you.

Should you remove the address?

Keep the address; fix the sending side

Keep it. A 421 is about your sending, not the address. Pausing the campaign is often the right move; deleting recipients is not.

Soft bounce. Your sending server retries on its own. Investigate only if it keeps failing.

How it appears

421 4.7.0 Try again later, closing connection

421 4.7.0 [TSS04] Messages from 203.0.113.5 temporarily deferred

421 4.7.1 Greylisting in effect, please come back later

What causes it

Greylisting — the server temporarily rejects unfamiliar senders on the assumption spam software will not retry.
Rate limiting, because you opened too many connections or sent too fast for that server's policy.
The recipient server is under load, or briefly misconfigured.
Your sending reputation is borderline, and the server is deferring rather than refusing outright.

How to fix it

In order — try the first one first.

Let the retry happen. A properly configured sending server retries automatically over several hours.
If it is systematic, slow down. Concurrency and send rate are the usual culprits, not the content.
Make sure your HELO name, reverse DNS and SPF record all agree — mismatched identity is the fastest route to being greylisted by everyone.
For mailbox verification specifically, a 421 is not a verdict on the address. It means unconfirmed, and an honest verifier reports risky rather than guessing.

Who sends it

Gmail uses it for reputation, PTR and TLS problems. Yahoo's TS-series deferrals, such as TSS04, also arrive as 421 4.7.0.

Gmail
Reply text
This message is suspicious due to the very low reputation of the sending IP address. To protect our users from spam, email sent from your IP address has been temporarily rate limited.
Gmail has a matching text for low domain reputation, and one for suspicious content or links.
Source: Google Workspace Admin Help: Gmail SMTP errors and codes
Gmail
Reply text
The IP address sending this message does not have a PTR record, or the corresponding forward DNS entry does not point to the sending IP.
Reverse DNS missing or mismatched.
Source: Google Workspace Admin Help: Gmail SMTP errors and codes
Yahoo
Yahoo groups its temporary deferrals as "TS* Errors" and lists complaints, spam-like content, poor IP reputation and unusual traffic as causes. See the TSS04 page.
Source: Yahoo Sender Hub: SMTP Error Codes

Common questions

What does SMTP 421 4.7.0 mean?: SMTP 421 4.7.0 is a temporary deferral: the recipient's server is refusing the message for now and asking you to retry. It is the most common code behind greylisting and rate limiting, and legitimate mail usually arrives on the retry minutes later without any action from you.
Should I retry a 421 4.7.0 error?: Yes, and your sending server does it automatically. 421 is a temporary deferral, and legitimate mail usually arrives on a retry within minutes or hours. Only investigate if it persists across many retries.
How do I fix SMTP 421 4.7.0?: Let the retry happen. A properly configured sending server retries automatically over several hours.
Is 421 4.7.0 a hard bounce or a soft bounce?: 421 4.7.0 is a soft bounce. It is temporary, and your sending server retries it automatically. Your sending server retries on its own. Investigate only if it keeps failing.
Should I remove the address after a 421 4.7.0 bounce?: Keep it. A 421 is about your sending, not the address. Pausing the campaign is often the right move; deleting recipients is not.
Will verifying my list prevent 421 4.7.0?: Only partly. Temporary deferrals are not about the address, but providers defer senders whose mail looks risky, and a high rate of hard bounces is one of the signals they use.

Clean the list before the next send.

A bounce report only tells you about the addresses you already mailed. The rest of the list has the same kinds of dead mailboxes in it, and each one becomes a hard bounce recorded against your domain the moment you send.

A real SMTP mailbox check on every row, before anything is sent
Dead and mistyped addresses split out, so they never reach your sending tool
Catch-all, disposable and role addresses flagged alongside

Get started — $39/mo See the published limits first

Plans from $9 a month, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.

Other 421 replies

Sources

Provider wording is quoted only where the provider publishes it.

Last reviewed 2026-10-22