Gmail "421-4.7.28 ... has exceeded its quota for sending" (IP rate limited)

What’s actually happening

Mail to Gmail and Google Workspace addresses starts deferring while other providers still accept it fine. The 421 is temporary, so messages queue rather than bounce, but the queue grows faster than it drains. Push harder and Gmail tightens the limit instead of loosening it. Often hits right after a list blast or a new IP goes live.

Common causes

• Sending volume from one IP or domain spiked far above its recent baseline in a short window
• A cold IP with no sending reputation pushing high volume before it has warmed up
• Spam complaints or spam-trap hits dragging down domain/IP reputation, lowering the quota Gmail allows
• Many concurrent SMTP connections or messages-per-connection from the same source, which Gmail reads as abuse
• A shared IP where another tenant's bad behavior is throttling everyone on the same address

How to fix it

1. Cut the send rate immediatelyLower concurrent connections and messages per minute to Gmail. In Postfix, add a transport with `smtp_destination_concurrency_limit` and `smtp_destination_rate_delay` (e.g. `1s`) scoped to gmail.com / googlemail.com via `transport_maps`. Honor the 421 with backoff instead of hammering.
2. Warm the IP up graduallyOn a new or rotated IP, start with hundreds of messages a day and roughly double every few days over 2-4 weeks. Send your most-engaged recipients first so early signals are positive.
3. Fix authentication and alignmentConfirm SPF passes, DKIM signs with an aligned d= domain, and DMARC is published. Check Google Postmaster Tools (postmaster.google.com) for your domain's reputation, spam rate, and authentication results.
4. Drive down spam complaintsPurge unengaged and bounced addresses, use confirmed opt-in, and add one-click List-Unsubscribe (RFC 8058). Keep the Postmaster Tools spam rate under 0.1%; above 0.3% Gmail throttles aggressively.
5. Split or dedicate IPs by traffic typeDon't mix bulk marketing and transactional mail on one IP. Move transactional to its own address so a marketing spike can't defer password resets and receipts.