How to know if your IP is being rate-limited or throttled?

You sent a campaign, and now your delivery report shows emails sitting in the queue for hours. Some arrive late. Others bounce back with cryptic messages like "421 Too many connections" or "Try again later." Sound familiar? That's what rate-limiting looks like in practice.

Quick distinction before we dig in: rate-limiting is when a receiving server caps how many messages it accepts from your IP per hour or per connection. Throttling is the same idea, just applied more broadly to your overall sending speed. Both result in deferred emails, not bounced ones. Your messages aren't rejected, they're just made to wait.

Step 1: Look at your bounce and deferral logs in your ESP

Your first stop is your sending platform's activity log or delivery report. Look for messages with a "deferred" or "temporarily failed" status rather than a hard bounce. In Mailgun, check the Logs tab and filter by "Temporary Failure." In Twilio SendGrid, go to Activity and filter for "Deferred." In Postmark, look at the Message Stream's delivery stats for delayed sends.

Now the key is the error code attached to each deferred message. Anything starting with 4xx is a soft, temporary rejection. The message that follows the code tells you exactly why.

Common 4xx messages that signal rate-limiting:

• "421 Too many connections from your IP" (volume-based limit)
• "421 Message rate limit exceeded" (per-hour cap hit)
• "450 Requested mail action not taken" (generic deferral, often reputation-related)
• "452 Too many recipients" (per-message recipient cap)

Step 2: Check Gmail and Microsoft's own tools

Gmail's Postmaster Tools shows a "Delivery Errors" graph under the domain's dashboard. A spike in error volume that coincides with your send time is a strong indicator that Google is slowing you down rather than blocking you outright. Postmaster Tools also shows your domain reputation and IP reputation in separate tabs. If your IP reputation sits at "Low" or "Bad," throttling is almost certain.

For Microsoft, look at Outlook's Smart Network Data Services (SNDS). A yellow or red traffic light next to your IP means reduced trust. You'll also see complaint rates and trap hit data there, which helps you understand why the throttling started.

Step 3: Look at your delivery time distribution

Normal delivery happens within minutes. If your ESP's reports show emails sent at 9am arriving between noon and 4pm, that's a queue backup caused by throttling. Pull a delivery time histogram if your ESP supports it. A long tail on delivery time is the clearest symptom, even when error codes aren't obvious.

Step 4: Check whether it's one domain or all of them

Filter your deferral logs by recipient domain. If the deferrals are concentrated at Gmail addresses or Microsoft addresses (Outlook, Hotmail, Live), the problem is specific to that provider's limits or your reputation with them. If you're seeing deferrals across all recipient domains, the issue is more likely your sending volume or IP reputation more broadly.

What normal vs. abnormal looks like

But a healthy send will have a deferral rate below 2% for most senders. If you're seeing 10%, 20%, or more of your messages deferred on a given send, something is actively wrong. Shared IP users on platforms like Mailchimp or Brevo sometimes see this after a spike in complaints from other senders on the same pool (not your fault, but still your problem).

If you're stuck on what the error messages mean or you can't find where to look in your ESP's dashboard, our SOS hotline is free and we actually pick up.