Fixing Gmail "421 4.7.28 unusual rate of unsolicited mail"

421 4.7.28 is a deferral, not a rejection: Gmail is temporarily throttling your sending IP and your server will keep retrying. It is a reputation and volume signal — Gmail thinks the IP is sending more than its reputation warrants.

What it means

The 4 class means try again later. Gmail slows a source when its complaint rate rises or when a cold IP sends a sudden burst. Most of the deferred mail eventually delivers, but throughput stays capped until reputation recovers.

Why it happens

Sending a large campaign from an IP with little history, a jump in spam-marks, or hitting spam-traps all pull the reputation down and trigger the throttle.

Diagnose it

Check the IP against blocklists to rule out a listing, and confirm authentication and rDNS are clean so Gmail trusts the source. Then watch your Google Postmaster Tools reputation dashboard.

Fix it

1. Slow down: ramp volume gradually and spread large sends over time rather than in one spike.

2. Cut the complaint rate — mail only engaged recipients and honour unsubscribes.

3. Make sure SPF or DKIM aligns and publish DMARC (required for bulk senders).

4. Monitor reputation in Postmaster Tools until the deferrals clear.