421 4.7.28: Unusual rate of unsolicited mail from your domain
421 4.7.28 from Gmail means it has seen an unusual rate of unsolicited mail coming from your domain or IP and is holding your messages back. It is temporary, and it is a clear sign to cut volume and check complaints.
- Bounce type
- Soft bounce (temporary)
- Usually caused on
- Your sending side
- Terlido report label
- Blocked by the provider
What the standard says
RFC 5321 defines 421 as "service not available, closing transmission channel". The enhanced code X.7.28 has no definition in RFC 3463 for this use, so treat it as specific to Gmail. Subject 7 means a security or policy reason, and class 4 means the sender should try again later.
What the mailbox providers say
- Gmail (421 4.7.28): “Gmail has detected an unusual rate of unsolicited email originating from your DKIM domain...” Source
Common causes
- A sudden spike in volume from a domain that used to send little.
- A high rate of spam complaints from recipients.
- A new domain that sends too much too early, before it has a record of normal mail.
- A list that holds many people who did not expect your message.
- Gmail names the part it linked to the problem: the DKIM domain, the IP address, the SPF domain, or the domain in a URL. Read which one it names.
- Sending to a list that was bought or scraped, where many recipients do not know you and mark the message as spam.
How to fix it
- Reduce the daily volume now and keep it low until the deferrals stop.
- Review your recent campaigns for complaints and remove the contacts who complained.
- Tighten the audience so only relevant people receive each message.
- Check your authentication and fix anything that fails.
- If Gmail names a URL domain, check the links in your copy and remove tracking or shortener domains with a poor record.
- Stop sending to the part of the list that never opened or replied, and keep to people who have a reason to hear from you.
- Raise volume only in small steps after the deferrals stop, and watch the next day before you raise it again.
Is it a deliverability problem?
Yes. This code means Gmail already judges your sending pattern as unwanted. It is a reputation signal, and it can lead to permanent blocks if the pattern stays. Slow down first, then work on the list and the content. Gmail is describing a rate, which means the problem is the shape of your sending and not one single message. Fixing the copy alone will not help if the audience is wrong. Cut the volume first, then clean the list, then review the message, in that order.
How Terlido handles it
When a bounce message for this code arrives in a connected mailbox, Terlido reads the status code. An enhanced code that starts with 4 is a soft bounce. In the campaign Reports tab this one is grouped as "Blocked by the provider".
On a soft bounce Terlido logs the event on the lead's activity. The lead is not stopped, and the sequence continues at its next scheduled step.
Check the domain behind the bounce
The free domain checker runs live SPF, DKIM, DMARC, MX and blocklist lookups and explains each result in plain English.
Free tools that help
- Gmail & Yahoo bulk sender requirements checkerReview live DNS and your sending practice against Google's published sender guidance.
- Cold email sending volume calculatorEstimate first touches and total email capacity while reserving room for warmup and follow-ups.
- Email warmup schedule calculatorSee Terlido's warmup ramp day by day: 5 messages on day one, about 10% more each day, never more than 3 extra, up to your target.
Related error codes
- 421 4.7.0: Temporarily deferred, often for low reputation
- 421 4.7.26: Rate limited because mail is unauthenticated
- 550 5.7.1: Delivery not authorized, message refused