How can you tell if Gmail rate limits are in effect?

Your Gmail delivery rate suddenly drops, but every other provider looks fine. Sound familiar? That's one of the clearest signs that Gmail is rate limiting your sends rather than rejecting them outright.

The most direct signal is a 421 error code in your bounce logs. Gmail's version typically reads something like "421-4.7.0 Try again later" or "421 Slow down, too fast." These aren't hard bounces. They're temporary rejections telling your sending server to back off and retry. Your ESP should retry automatically, but if the 421s keep stacking up, emails sit in a queue for hours before any reach the inbox.

Here's what to look for across your tools:

• Bounce logs: 421 codes tied to Gmail addresses specifically, while other domains show normal delivery.
• Delivery timing: Emails accepted by Gmail but taking three to eight hours to appear in inboxes instead of minutes.
• Gmail Postmaster Tools: A spike in the Delivery Errors graph is your clearest dashboard signal. If you're not using Gmail Postmaster Tools yet, this is exactly the situation where it earns its keep.
• Sending rate vs baseline: Rate limiting often kicks in when volume jumps suddenly, when you're sending fast from a new or low-reputation IP, or when your domain reputation has slipped.

The fix isn't glamorous. Slow down. Spread the same volume over a longer window. If you're on new infrastructure, that means a proper warm-up sequence before pushing volume hard. Gmail's rate limits are tied to your reputation level, so senders with stronger engagement history get more headroom (which connects directly to how Gmail's reputation model works).

If you're stuck mid-send and not sure whether you're looking at rate limiting or something worse, our SOS hotline is free and we actually pick up.