What it looks like in your logs
421-4.7.28 [203.0.113.10] Gmail has detected an unusual rate of unsolicited mail originating from your DKIM domain [example.com]. To protect our users from spam, mail sent from your domain has been temporarily rate limited. - gsmtpWhat does 4.7.28 mean?
The IANA registry lists X.7.28 as mail flood detected. In its temporary form, 4.7.28, the receiver is throttling rather than blocking. Gmail is by far the biggest user and returns 421 4.7.28 when it sees an unusual rate of unsolicited mail. Your MTA backs off and retries, which is why the problem can hide behind eventual delivery. Delivered eventually does not mean healthy.
The useful part is which signal Gmail names. The text can refer to your IP address, your IP netblock, your DKIM domain, your SPF domain or a URL domain in the message body. If it names the DKIM domain, moving to new IPs will not help because the limit follows your domain. If it names a URL domain, a link in your content is the problem. If it names a netblock, a neighbor on shared hosting may be involved.
A sustained 4.7.28 rate is the earliest actionable warning Gmail gives, and senders who ignore it because mail eventually arrives often meet the permanent version next. Left alone, it tends to become a permanent 5.7.28 within days. Track deferrals by provider and by the named signal so you can see the trend early and respond before the block hardens.
Soft or hard bounce?
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| Sender behaviour | Queue and retry with backoff | Stop, suppress the address |
| Reputation risk | Low, unless it repeats for days | High above a 2% bounce rate |
| 4.7.28 | ✓ this code |
Common causes
- Sending volume to Gmail rose faster than your reputation supports.
- Complaints or spam-folder placement pushed IP or domain reputation down.
- A new IP or domain started sending at volume without warmup.
- A shared netblock is throttled because of other senders on it.
- A link or redirect domain in your content has poor reputation.
- Messages were sent with duplicate or malformed Message-ID headers.
How to fix 4.7.28
- Read the deferral text and note which signal it names: IP, netblock, DKIM domain, SPF domain or URL domain.
- Cut volume to Gmail immediately and send only to recently engaged recipients until deferrals fall.
- Check spam rate in Google Postmaster Tools and the sender reputation checker; keep complaints below 0.1% and never above 0.3%.
- If a URL domain is named, remove or replace that link, shortener or tracking domain.
- Make sure every message has a unique, well-formed Message-ID header.
- Ramp back up over two to four weeks using the IP and domain warmup guide.
4.7.28 questions
What does 421 4.7.28 mean from Gmail?
It means Gmail has noticed an unusual rate of unsolicited mail from you and is temporarily limiting how much it accepts. It is based on complaints, spam placement and volume patterns. The text says whether the limit applies to your IP, domain or a link domain.
How long does a 4.7.28 rate limit last?
There is no fixed timer. Deferred messages often deliver on retry within hours, but the limit continues until the reputation signals improve. After you reduce volume and clean the list, most senders see it ease within several days to two weeks.
Is 4.7.28 the same as 5.7.28?
They share a meaning but not a severity. 4.7.28 is a temporary rate limit and your mail is retried. 5.7.28 is a permanent block. A rising 4.7.28 rate is usually what comes before it. Slow down well before that happens.
Why does 4.7.28 continue after I moved to a new IP?
Because the limit is probably on your DKIM or SPF domain, not the IP. Gmail tracks domain reputation separately, and it follows you to any IP. Fix list quality and complaints rather than rotating infrastructure, which can also look like evasion.