421-4.7.28 Gmail Unusual Rate of Unsolicited Mail (IP Rate Limited)

What’s actually happening

Messages to Gmail/Workspace recipients stop going through and your queue fills with 421-4.7.28 deferrals. The 4.x.x class means temporary — Gmail is throttling, not bouncing, so the mail keeps retrying and may eventually deliver or time out. Delivery to non-Gmail domains usually still works, which points the finger at your reputation with Google specifically. The error sometimes names your DKIM domain, SPF domain, or a URL in the message instead of the IP.

Common causes

• A genuine volume spike from your IP — a campaign blast, a new sending pattern, or a host you share an IP with — tripped Gmail's rate limiter.
• Weak or failing authentication: missing/misaligned SPF, DKIM, or a DMARC record, so Gmail can't tie the mail to a trusted identity.
• Poor list hygiene — sending to stale addresses, spam traps, or recipients who didn't opt in — driving spam complaints up.
• A compromised account or script on your network sending spam through the same IP, dragging the whole IP's reputation down.
• Sending from a shared/host IP whose reputation was damaged by another tenant.

How to fix it

1. Pause and slow the send rateStop the blast. 4.7.28 is a soft deferral that clears as your reputation recovers, so resume at a much lower volume and ramp up gradually rather than hammering the queue, which only reinforces the throttle.
2. Fix authentication firstPublish and align SPF, DKIM, and DMARC for the sending domain. Confirm with a test message to a Gmail account and check Show original — SPF, DKIM, and DMARC should all read PASS. Google's 2024 sender rules treat missing auth as a strong spam signal.
3. Clean the list and cut complaintsRemove bounces, stale and unengaged addresses, and anything you can't prove opted in. Make unsubscribe one click and honor it fast. In Google Postmaster Tools, keep the spam-complaint rate under 0.3% (ideally below 0.1%).
4. Find any compromised sender on the IPAudit who/what is sending through that IP. Look for a hacked mailbox, an open web form, or a rogue script blasting mail. On a shared host, ask the provider whether a neighbor torched the IP and request a dedicated IP if so.
5. Monitor reputation in Postmaster ToolsAdd the domain to Google Postmaster Tools and watch IP/domain reputation, spam rate, and authentication results. Wait for reputation to climb back to Medium/High before returning to normal volume.