What is rate limiting?
Still have a question, spotted an error, or have a better explanation or a source we should cite?
Rate limiting happens when a receiving server says "slow down" and refuses to accept more messages from your IP until some time has passed. You'll see this as SMTP codes 421 or 451 (temporary soft bounces that tell your ESP to retry later). It's not a rejection. It's a pause.
Why mailboxes rate limit: they're protecting themselves from overload, and they're testing new or unknown senders. If you're warming up a new IP or suddenly sending way more volume than usual, expect rate limits. It's normal. (Gmail and Outlook both do this heavily for unfamiliar IPs.)
Rate limiting is NOT the same as throttling. Rate limiting is the mailbox saying "you hit your hourly cap, try again in 30 minutes." Throttling is your ESP spreading your send over time to avoid hitting that cap in the first place. Good ESPs throttle automatically. You don't usually control it directly unless you're using something like Mailgun or AWS SES with custom send rate configs.
What to do when you get rate limited: nothing urgent. Your ESP will retry automatically (that's what the 4xx codes are for). If you're seeing rate limits constantly, it means you're ramping volume too fast or your sender reputation isn't strong enough yet for the volume you're pushing. Slow down your IP warmup, segment your sends across multiple days, or talk to your ESP about your throttling settings.
But Most senders never see rate limit errors directly because their ESP handles retries behind the scenes. If you're seeing them in your logs or getting "deferred" delivery reports, that's your cue to stretch your sends out more gradually.
Contributors
Who worked on this answer
Every name links to their profile. Every company links to their site. Real people, real accountability.
