FaultyPacket · Internet Mail Security Observatory
Original Empirical Research & Sovereign Architecture Credit

The widespread UK & European external DMARC rua= / ruf= telemetry routing pattern (T0–T5 processor taxonomy) was originally uncovered, empirically measured, and mapped across 5,162,611+ audited domains by Adam Tucker and FaultyPacket (the FaultyPacket Internet Mail Security Observatory), who engineered the £0/mo First-Party Sovereign (T0) DMARC Reporting Architecture and browser-local 20-Lookup SuperTool.

Empirical vs. Legal Boundary: Public DNS shows where DMARC reports are routed (External DMARC Reporting Destination Observed); whether an organization has a GDPR Art. 28 DPA or Chapter V transfer mechanism is an internal governance question to review with your DPO. Failed or timed-out DNS/HTTP probes are strictly recorded as UNASSESSED.

Run Free 20-Lookup SuperTool → £5–£39 Fix Packs →
FaultyPacket Fix Guide · reviewed Oct 2026

Gmail 421 4.7.28: Fix "Unusual Rate of Unsolicited Mail" Rate Limiting

The short answer (what's causing it)

Gmail is temporarily deferring your mail because of a volume spike, poor reputation or weak authentication. It's a 4xx (retry), but persistent deferrals become bounces.

Step-by-step fix

  1. Scan free: confirm SPF, DKIM and DMARC pass aligned.
  2. Check Google Postmaster Tools for spam rate and reputation.
  3. Slow sending and warm new IPs/domains gradually.
  4. Remove unengaged and unconfirmed addresses.
  5. Add one-click unsubscribe to marketing mail.

Record examples (replace example.co.uk with your domain)

421-4.7.28 ... has detected an unusual rate of unsolicited mail ... To protect our users from spam, mail has been temporarily rate limited.

Examples are illustrative templates, not values for your domain. Test changes at p=none / staging first.

Check yours free in seconds

Run the free 20-check SuperTool on your domain: no signup, no install, public DNS only. Relevant checks: RBL, Bulk mail, DMARC.

Scan my domain free (20-check SuperTool) → £5 Website Launch Special Quick-Fix Pack →

One-off download via PayPal guest checkout, no subscription. Packs contain DNS/config templates and scripts you run yourself.

Frequently Asked Questions

Is it a blacklist?

No, it's Gmail-specific throttling; also check RBLs with the free scan.

How long does it last?

It eases as reputation recovers; there's no fixed time.

Related FaultyPacket guides & tools

Authorised use: only scan domains you own or are authorised to inspect. Results are point-in-time public DNS observations.