What is “rate limited”?
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You hit send on a big campaign, and instead of clean delivery you start seeing deferred messages piling up. The error says something like "421 rate limit exceeded" or "please slow down." That's rate limiting. It's the receiving server telling you to queue up and wait your turn.
Rate limiting means the destination mail server is accepting your messages, just not at the speed you're sending them. Nothing is permanently rejected. Your emails are deferred and retried. A good sending platform handles those retries automatically, but the delay can stretch from minutes to hours depending on the server's patience and your sending reputation.
Servers rate limit for a few reasons. High volume in a short window strains their infrastructure, so they throttle everyone past a certain threshold. Newer senders or domains with lower sender reputation get tighter limits than established ones. And if a sending IP is showing patterns that look like spam, the limits get tighter fast.
The SMTP codes you'll see most often are 421 (try again later), 450, and 452 (mailbox or resource temporarily unavailable). Microsoft 365 has its own flavor with codes like "RP-001" attached to rate limit messages. These are all soft bounces in practice. They don't hurt your list the way a hard bounce does, but too many in a short window can still signal trouble to the receiving server.
What to do when you're rate limited:
- Slow down your sending rate. Most ESPs let you cap messages per hour or per connection.
- Spread large campaigns over a longer window instead of blasting all at once.
- Warm up new IPs or domains gradually so receiving servers learn to trust you before you push volume.
- Check whether your reputation is the real issue. Rate limits that tighten over time, rather than staying flat, usually signal a reputation problem rather than a simple volume one.
Rate limiting isn't a disaster. It's a signal worth listening to. If you keep hitting it on the same provider, it's worth checking your blocklist status too.
You can check your domain's reputation and blocklist standing with our free blocklist checker. Or if you're stuck trying to figure out why a specific provider keeps throttling you, our SOS hotline is free and we actually pick up.
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