Is 421 4.7.28 permanent or temporary?

421 4.7.28 is a temporary (4.x.x) failure — a properly configured mail server retries automatically, and the message often delivers on a later attempt. Persistent 421 4.7.28 across recipients points to a deeper issue.

Is this my problem or the recipient's?

Often it resolves itself on retry. If it persists across many recipients, it's usually a sending-side reputation or authentication issue worth fixing.

How do I check my sending setup?

Run a free EmailHealth check on your domain — it verifies SPF, DKIM, DMARC, blacklist status, and more in a couple of seconds, which covers the most common causes of delivery failures.

421 4.7.28 — Gmail: unusual rate of unsolicited email

421 4.7.28 is a temporary SMTP reply meaning gmail: unusual rate of unsolicited email. This is a reputation problem, not a DNS problem. Too many recipients mark the mail as spam, or the volume jumped faster than your reputation allows.

What 421 4.7.28 means

This is a reputation problem, not a DNS problem. Too many recipients mark the mail as spam, or the volume jumped faster than your reputation allows.

ℹ️

This is a 4.x.x temporary failure — a correctly configured mail server retries on a backoff schedule, and the message frequently delivers on a later attempt.

Common causes

📨

Spam complaints are too high

Most common

Google asks bulk senders to keep the user-reported spam rate below 0.3% in Postmaster Tools, and ideally below 0.1%.

🔁

Volume jumped on a new or cold domain

Most common

A new domain or IP that suddenly sends thousands of messages looks like a spam run.

⚙️

The list has old or bought addresses

Common

Low engagement and spam traps lower reputation quickly.

How to fix 421 4.7.28

1
Slow down and back off
Lower the volume to Gmail right away and let your server retry slowly. Pushing harder makes the block longer.
2
Check Google Postmaster Tools
Look at the spam rate and compliance status for your domain. Fix the source of complaints first.
3
Send only to engaged, opted-in people
Remove inactive and unverified addresses, add one-click unsubscribe, and ramp volume up slowly.

Confirm your domain is set up correctly

Most delivery failures — permanent or temporary — trace back to authentication or reputation. Run a free EmailHealth check to confirm SPF, DKIM, DMARC, blacklist status, and TLS are all healthy for your sending domain. If you send in volume, free daily monitoring will alert you before a small issue becomes a flood of bounces.

Gmail: unusual rate of unsolicited email