How do throttling rules affect automated sends?

You trigger a welcome sequence, a cart abandonment flow fires at the same time, and suddenly half your contacts get emails hours late. Is your ESP slowing you down? Is Gmail pushing back? Or is something else going on entirely? Throttling is usually the culprit, but the fix depends on which kind is hitting you.

There are three distinct types, and they behave very differently in the wild.

ESP-level throttling is a limit your own platform sets. Most ESPs cap how many messages you can send per hour or per day, partly to protect shared infrastructure, partly to protect your reputation. If you're on Mailchimp or Brevo and you fire a large automation at peak time, you'll hit this wall first. You'll see it in your send logs as delayed sends queuing up, not as bounces or deferrals from the receiving end.

ISP-level throttling is what happens on the receiving side. Yahoo Mail and Outlook both enforce incoming rate limits. When you exceed them, the receiving server issues a temporary deferral with a 4xx error code and says, in effect, "slow down and try again later." Your ESP's mail queue then retries delivery over the next few hours. The symptom here is delayed delivery accompanied by 4xx codes in your bounce logs, not hard failures.

Reputation-based throttling is the sneaky one. New sending domains, new IPs, or senders recovering from a complaint spike all get stricter rate limits regardless of volume. Mailbox providers don't announce this. They just accept fewer messages per connection until they trust you more. The fix is a proper IP warm-up plan, not just slowing your sends down arbitrarily.

How to tell which one you're dealing with

• Check your ESP's sending logs first. If messages are queuing before leaving your ESP at all, that's an ESP-level cap.
• If messages leave your ESP but show 4xx deferrals in delivery logs, the ISP is throttling you. Look for error messages like "421" or "451" with phrases like "too many connections" or "try again later."
• If you're seeing lower-than-expected open rates with no obvious 4xx trail, reputation-based throttling may be filtering your delivery quietly. Check your domain reputation with our free blocklist checker as a starting point.

What you can actually do about it

• For ESP caps, check your plan limits and spread large automation triggers across smaller batches or off-peak hours.
• For ISP deferrals, don't panic. Your ESP's retry logic handles most of these automatically. If delays persist beyond 24 hours, volume reduction or a sending pause usually helps.
• For reputation-based limits, slow and steady volume growth is the only real answer. Sending cleaner lists helps too. (A list clean before a high-volume automation run is worth it if your list has been sitting for a while.)
• Time-sensitive transactional messages like password resets should live on separate sending infrastructure from your marketing automations. Don't let a promotion campaign's throttling hold up a receipt.

If you're in the middle of this right now and not sure what your logs are telling you, our SOS hotline is free and we actually look at the details with you.