Why It Matters
Blast 100,000 emails from a new IP in one hour and you'll almost certainly get blocked. ISPs like Gmail, Yahoo, and Outlook set receiving limits per sending IP and domain. Exceed those limits and your emails get deferred (temporarily rejected) or outright blocked. Rate limiting is how you play nice with ISP infrastructure and keep your messages flowing.
How It Works
Rate limiting can be imposed at multiple levels. ISPs set inbound rate limits -- Gmail, for instance, will defer messages from an IP that sends too much too fast, returning temporary 4xx errors. Your email platform or MTA also has outbound rate controls that spread delivery across minutes or hours instead of firing everything at once.
During IP warming, rate limits are especially critical. You typically start with a few hundred emails per day and gradually ramp up over 2-4 weeks. The exact ramp depends on your volume goals, but doubling every 2-3 days is a common pattern.
Quick Tips
- If you're seeing lots of 4xx deferral errors, you're likely hitting the ISP's rate limit -- slow down and spread sends over a longer window
- Use your ESP's built-in throttling features rather than trying to manage rate limits manually
- Separate your transactional and marketing email streams so a marketing blast doesn't cause rate limiting for your password reset emails