Gmail "421 4.7.28 ... unusual rate of unsolicited mail" deferrals: how to fix

What's happening: Gmail is temporarily deferring your mail, not bouncing it, because it saw a spike of messages from your IP or domain that recipients treat as unwanted. Your server will usually retry automatically. Keep hammering and the 421 can turn into a hard 550 5.7.1 rejection.

How to fix it

  1. Slow down at once: cut your sending rate and let queued mail retry on its normal schedule instead of resending.
  2. Check that SPF, DKIM and DMARC pass and align with your From domain. Unauthenticated senders hit this limit much sooner.
  3. Look at what changed: a new list, a sudden volume jump, or a new campaign. Remove addresses that never opened or that came from old or bought lists.
  4. Add one-click unsubscribe (List-Unsubscribe with RFC 8058) and keep complaints under 0.3% in Google Postmaster Tools.
  5. Ramp back up gradually over days, not hours. Warm up new IPs or domains with your most engaged recipients first.

Not sure which record is wrong? Check your domain free: it shows which of SPF, DKIM and DMARC is failing, in 5 seconds, no signup.

Other fix guides