Gmail 421 4.7.28 'Unusual Rate of Unsolicited Email': What It Means and How to Fix It
421 4.7.28 Gmail has detected an unusual rate of unsolicited email originating from your DKIM domain means Gmail is deferring your mail because it has seen a spike in email, or in unwanted email, from one thing it can identify about you. The variant names that thing: your IP address, your IP netblock, your DKIM domain, your SPF domain or a URL domain in the message. It is temporary, so your server will retry. If you keep sending at the same rate, it does not go away, and it can turn into the permanent 550 5.7.28. The fix is to cut volume to the named sender, confirm your authentication, clean the list and stop whatever is generating spam reports.
The variants, from Google's own list
Google's Gmail SMTP errors and codes page (last updated 26 October 2026, read 2 November 2026) lists these messages under 421 4.7.28:
| Message text | What Gmail is judging | Where to look |
|---|---|---|
| "Gmail has detected an unusual rate of email." | Your sending rate in general | Daily volume, bursts |
| "...unusual rate of email originating from your IP address" | One sending IP | The IP in the bounce |
| "...unusual rate of unsolicited email originating from your IP Netblock" | A range of IPs, often a provider's | Your ESP or cold email platform |
| "...unusual rate of unsolicited email originating from your DKIM domain" | The domain in your DKIM signature | That domain's inboxes and campaigns |
| "...unusual rate of unsolicited email originating from your SPF domain" | The envelope (Return-Path) domain | The domain your platform uses for bounces |
| "...unusual rate of unsolicited email containing one of your URL domains" | A link in your message | Tracking domains, shorteners, your site |
| "...unusual amount of unsolicited email originating from your IP address." | Volume of unwanted mail from one IP | The IP in the bounce |
The permanent sibling is 550 5.7.28: "There is an unusual rate of unsolicited email originating from your IP address." That one is a rejection, not a deferral.
Not sure which one you have? Paste the full bounce into Bounce Doctor. It reads the code and the text, not just the number.
Why cold senders hit it
Gmail does not publish its thresholds. Its sender guidelines say two things that explain most cases: "Send email at a consistent rate. Avoid sending email in bursts", and "Avoid introducing sudden volume spikes if you do not have a history of sending large volumes."
The cold-email patterns that cause it:
| Pattern | Why Gmail notices |
|---|---|
| A new domain sent at full volume on day one | No history, sudden volume |
| Ten inboxes on one domain, each at the "safe" limit | Gmail sees the domain total, not the inbox |
| A campaign launched to a whole list at 9am | A burst, not a steady rate |
| Poorly targeted lists | Spam reports, which is the "unsolicited" in the message |
| Shared tracking domain or link shortener | The URL variant: someone else's spam carries the same link |
| Old or unverified list | Invalid addresses and spam traps drag reputation down |
The DKIM domain variant is the most common one for cold senders. Most platforms sign with your domain, so every inbox you add on that domain adds to the same total. Our how many domains for cold email guide covers how to split volume, and cold emails per day per inbox covers safe per-inbox numbers.
4.7.28 vs the codes it gets confused with
| Code | Meaning | Fix |
|---|---|---|
| 421 4.7.28 | Unusual rate of (unsolicited) email | Slow down, fix the cause |
| 550 5.7.28 | Unusual rate of unsolicited email, rejected | Stop, fix targeting and complaints |
| 421 4.7.0 / 550 5.7.1 | "Very low reputation" of the sending IP or domain | Rebuild reputation. See Gmail low reputation |
| 421 4.7.26 / 550 5.7.26 | Unauthenticated | Fix SPF and DKIM. See Gmail 550 5.7.26 |
| 452 4.2.2 | Recipient's inbox is full | Retry later; suppress if it persists |
| 550 5.1.1 | Address does not exist | Suppress the address |
The full list is in our Gmail bounce codes reference.
Fix plan
1. Find what is named
Read the full bounce. If it names your DKIM domain, the problem is that domain's total volume and complaint rate, across every inbox and tool that signs with it. If it names an IP or netblock you do not own, it is your provider's. If it names a URL domain, it is a link in your message.
2. Cut volume to Gmail from that sender
Halve the daily send to Gmail and Google Workspace recipients from the named domain or IP, or pause it for a few days if deferrals are heavy. Keep any mail you do send steady through the day. Do not move the same campaign to a fresh domain: Gmail judges behaviour, and the new domain will follow the old one.
3. Check authentication
Make sure the domain passes SPF, DKIM and DMARC, and that the From domain aligns with at least one of them. Gmail rate-limits unauthenticated mail with separate codes, and weak authentication makes reputation harder to build. The SPF checker, DKIM checker and DMARC checker cover each one, and the bulk sender checker runs Google's requirements in one pass.
4. Look at the spam rate
Google's guidelines say to keep the user-reported spam rate in Postmaster Tools "below 0.10% and avoid ever reaching a spam rate of 0.30% or higher." Set up Google Postmaster Tools for the domain if you have not. Cold email rarely reaches the volume for full data, but when it does, the spam rate is the number to watch.
5. Fix the list and the targeting
"Unsolicited" means people are marking you as spam, or Gmail predicts they would. Tighten the targeting, shorten the sequence for non-responders, make opting out easy, and remove anyone who replied "not interested". Verify every unsent lead, because invalid addresses and spam traps are a reputation signal of their own, and suppress every hard bounce.
6. Fix the URL variant separately
If a URL domain is named, remove that link. Use your own custom tracking domain instead of a shared one, avoid public link shorteners, and send early touches with no links at all.
7. Ramp back slowly
When deferrals stop, increase volume by small steps over a week or two. Our warm-up schedule shows a sensible pace.
Can you ask Google to lift it?
Google's sender guidelines FAQ says bulk senders that meet all of its requirements can submit an escalation through the sender contact form. It also says "Bulk senders remain ineligible for mitigation while user-reported spam rate is greater than 0.3%", and that eligibility returns once the rate stays below 0.3% for 7 consecutive days. If you are below those levels and fully authenticated, an escalation is worth trying. If you are not, fix the cause first.
What not to do
- Do not resend deferred messages by hand. Your server is already retrying. Manual resends double the rate.
- Do not add more inboxes on the same domain. Gmail counts the domain.
- Do not buy aged domains to get around it. The pattern that caused the deferral comes with you.
- Do not ignore it. Repeated 4.7.28s are a warning before rejections and spam folder placement.
Where SimpleVerifier fits
Verification will not lift a rate limit. It removes one of the inputs. SimpleVerifier runs a live mailbox check on each address and returns safe to send, send with care, check again later or do not send, with the reason, so your next send to Gmail goes to addresses a server confirmed and fewer bounces feed into your reputation. Catch-alls stay risky, never guessed valid.
The first 100 checks are free with a Google sign-in on the verify page. Pro is $39 a month with the first 100,000 checks a month at full speed, then slower, never cut off. See pricing.
Common questions
What does 421 4.7.28 mean in Gmail?
Gmail has detected an unusual rate of email, or of unsolicited email, from something it can identify: your IP address, your IP netblock, your DKIM domain, your SPF domain or a URL domain in the message. It is a temporary deferral, so your server will retry, but repeated 4.7.28 errors mean Gmail is limiting how much mail it accepts from you.
Is 421 4.7.28 a hard bounce?
No. 421 is a temporary failure and your mail server should retry. If the message still cannot be delivered after the retry period, it becomes a bounce. The related 550 5.7.28 is a permanent rejection.
How long does a 4.7.28 block last?
Google does not publish a duration. The rate limit eases as Gmail sees lower volume and fewer complaints from the named IP or domain. Cutting volume sharply and fixing the cause is usually needed before deliveries return to normal.
Can I request removal from Gmail's 4.7.28 limit?
Google says bulk senders that meet all of its sender guidelines can submit an escalation through its sender contact form, but that senders stay ineligible while their user-reported spam rate is above 0.3%, until it stays below 0.3% for 7 consecutive days.
Does email verification fix 421 4.7.28?
Not directly. 4.7.28 is about rate and complaints, not bad addresses. Verification helps because invalid addresses and old lists are a common cause of poor reputation, and removing them lowers bounces that feed into it.
Check your list before you send
Real mailbox checks over SMTP, and nothing is sent to the address. Sign in with Google and your first 100 email checks are free, no card. Plans for bigger lists are on the pricing page.