Should I retry a 421 4.4.5 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.

Is 421 4.4.5 a hard bounce or a soft bounce?

421 4.4.5 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 4.4.5?

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 4.4.5

Server busy

421 4.4.5 means the receiving server is too busy to take your message right now and closed the connection. It is a soft bounce on their side. Your server retries automatically, and nothing about the address or your reputation is implied.

Should you remove the address?

Keep the address; fix the sending side

Keep it. Congestion on the receiving server says nothing about the address.

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

How it appears

Server busy, try again later.

What causes it

The receiving server is handling more connections than it can take.
Your server opened many connections to the same provider at once.

How to fix it

In order — try the first one first.

Let the queue retry.
If you open many connections at once to one provider, lower the concurrency.

Who sends it

Gmail documents it. Busy or overloaded servers anywhere send the same code.

Gmail
Reply text
Server busy, try again later.
Gmail closes the connection after this reply.
Source: Google Workspace Admin Help: Gmail SMTP errors and codes

Common questions

What does SMTP 421 4.4.5 mean?: 421 4.4.5 means the receiving server is too busy to take your message right now and closed the connection. It is a soft bounce on their side. Your server retries automatically, and nothing about the address or your reputation is implied.
Should I retry a 421 4.4.5 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 4.4.5?: Let the queue retry.
Is 421 4.4.5 a hard bounce or a soft bounce?: 421 4.4.5 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 4.4.5 bounce?: Keep it. Congestion on the receiving server says nothing about the address.
Will verifying my list prevent 421 4.4.5?: 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 — $29.99/mo See the published limits first

One plan, 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