Why It Matters
Blast 100,000 emails to Gmail in five minutes and you'll get throttled — or worse, temporarily blocked. ISPs set rate limits to protect their infrastructure and their users. If you exceed those limits, you'll see a wave of 421 or 452 SMTP errors (temporary failures) and your messages will queue up. Persistent abuse of rate limits can permanently damage your reputation with that provider.
How It Works
Throttling can happen on your end (intentionally) or the receiving end (as enforcement). On your side, you configure your sending platform to limit output — for example, no more than 500 emails per minute to any single domain. On the receiving side, ISPs impose their own limits. Gmail's limits for new IPs start very low (sometimes as few as 50-100 messages per hour) and increase as you build reputation. Microsoft has similar but different thresholds.
Quick Tips
- When warming up a new IP, start with 50-100 emails per day and increase by 50% every few days.
- If you see 421/452 errors, slow down immediately — don't keep hammering the server.
- Spread large campaigns over several hours rather than sending everything at once.
- Different ISPs have different limits. Research the specific thresholds for Gmail, Outlook, and Yahoo if they're your biggest recipient domains.