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.

Temporary — will retry

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.

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.

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.

Last reviewed 2026-09-11