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Gmail 421 4.7.28: unusual rate of email

How-to guide
For:
Publishers and marketers, Workspace admins
Needs permission:
providers:view, providers:managesensitive, campaigns:sendsensitive
In the app:
Sending providers, then Events & sync

Gmail 421 4.7.28 is a temporary refusal: Gmail has seen an unusual rate of email, often unsolicited, from your IP address, your domain or a link domain in your messages, and slows that mail down until the rate looks normal again. This page explains the reply and how to bring your sending back to a pace Gmail accepts.

What Gmail means

Section titled “What Gmail means”

Gmail sends the code with one of several texts. The most common one names the sending IP address:

Gmail has detected an unusual rate of email originating from your IP address ip-address. To protect our users from spam, email sent from your IP address has been temporarily rate limited.

The other variants name what Gmail is limiting:

  • your IP netblock (“unusual rate of unsolicited email originating from your IP Netblock”)
  • your DKIM domain or your SPF domain (“unusual rate of unsolicited email originating from your DKIM domain”)
  • a link domain (“unusual rate of unsolicited email containing one of your URL domains”)
  • your IP address, blocked for now instead of rate limited (“an unusual amount of unsolicited email originating from your IP address”)
  • repeated Message-IDs (“this sender exceeded the quota for sending messages that have the same Message-ID”)

Each variant points to Google’s email sender guidelines. Source: Gmail SMTP errors and codes, Google Workspace Help. Checked October 2026.

Why it happens

Section titled “Why it happens”

Gmail rate limits when your volume, or the share of your mail its users don’t want, grows faster than your sender reputation supports. Common causes:

  • A jump in volume from an IP address, domain or sending provider that hasn’t been warmed up.
  • Mail to people who didn’t ask for it or stopped reading it, which raises the spam rate. Google asks senders to keep the spam rate reported in Postmaster Tools below 0.3% (email sender guidelines, checked October 2026).
  • A link domain in your content that Gmail sees in a lot of unwanted mail (the URL-domain variant).

How ReputeCast classifies it

Section titled “How ReputeCast classifies it”
  • While the sending provider retries, each refused attempt arrives as a deferral: a Deferred event in the event log. Deferrals don’t appear in the campaign report’s bounces table.
  • Deferrals count toward the deferral rate of the Gmail mailbox provider during a send. Above 5.0% (fixed; it counts after 500 messages to Gmail and at least 5 deferrals, with about 90% confidence), ReputeCast auto-pauses the Gmail share of the campaign, or the whole campaign when the pause scope is set to the campaign.
  • If the sending provider stops retrying, it reports a bounce. Gmail’s text contains the word “spam”, so the bounce class is spam_block and the report row is Blocked · policy or reputation. A bounce with a 4xx reply is recorded as a soft bounce and doesn’t suppress the address, unless the sending provider itself calls it permanent (Amazon SES and Resend follow their own bounce type; see when ReputeCast suppresses an address).
  • The Message-ID variant, or the bare code without Gmail’s text, is temporary (Soft · deferral expired).

SMTP error codes explains the classifier.

Before you begin

Section titled “Before you begin”
  • providers:view to read the event log.
  • providers:manage to change the throttles in Settings, then Providers.
  • campaigns:send to resume an auto-paused send.

Recover from the rate limit

Section titled “Recover from the rate limit”
  1. Find the variant. Open Sending providers, then Events & sync, set Event type to Deferred and search for a Gmail recipient. Original code · diagnostic shows Gmail’s text, which says whether Gmail limits an IP address, a domain or a link.
  2. Slow down to Gmail. Gmail has no throttle by default. In Settings, then Providers, then Throttles per mailbox provider, set a Gmail rate in messages per second. If a send is already auto-paused, the live monitor recommends a lower rate for the Gmail share; resume with that change.
  3. Send to engaged readers first. Contacts who stopped opening raise the share of unwanted mail. Use the list health tiers and let Send Readiness exclude unengaged contacts when it warns about them.
  4. If the IP address, domain or sending provider is new, follow a warm-up plan instead of sending the full list at once.
  5. For the URL-domain variant, review the links in the campaign. Content checks flag shortened and blocklisted links; link to your own domain instead of a public link shortener.
  6. For the DKIM-domain and SPF-domain variants, watch your domain’s spam rate in Google Postmaster Tools. ReputeCast shows Postmaster data on Deliverability, then Health once it receives it; see monitor domain and provider health.

Check that it worked

Section titled “Check that it worked”
  • On the next send, the live monitor’s Gmail deferral rate stays low and the send isn’t auto-paused.
  • The event log shows delivered events for Gmail recipients instead of new 421 4.7.28 deferrals.

Troubleshooting

Section titled “Troubleshooting”
  • Deferrals continue for days. Lower the Gmail throttle further and send only to your most engaged contacts until the deferrals stop, then raise the volume step by step.
  • The event log shows the code without Gmail’s text. Some sending providers report only the code. The class is then temporary; the fix is the same.
  • Complaints are high as well. Read bounces, complaints and provider limits and check that every message has one-click unsubscribe.
Section titled “Related”
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