What is email throttling?
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You hit send. The messages go out. But they don't arrive all at once. Some land right away, others trickle in over hours, and a few don't show up until the next day. That's often throttling at work.
Throttling is when a receiving mail server deliberately slows down how fast it accepts messages from your domain or IP address. The messages aren't rejected. They're queued and delivered in smaller batches, usually over minutes or hours.
Mailbox providers throttle for a few reasons. The most common is volume control: if a sender suddenly sends 500,000 messages in an hour, the server needs to manage the load. But throttling is also used as a reputation signal. A server that doesn't recognize your IP or domain, or that has seen inconsistent behavior from you in the past, may slow-walk your messages while it evaluates whether you're a legitimate sender.
How do you know you're being throttled?
Your ESP logs will show deferred messages, often with a message like "421 Try again later" or "450 Requested mail action not taken." This is different from a hard bounce (permanent failure) or a block (outright rejection). Deferrals mean: we're accepting this, just slowly.
Throttling is more common when you're sending to a new domain, warming up a new IP, or when you've had a recent spike in complaints or bounces. New senders hitting IP warming almost always experience some throttling as providers get used to seeing their traffic.
The underlying fix is usually the same: build sender reputation over time, clean your list so your bounce and complaint rates stay low, and make sure your authentication is set up correctly. Throttling usually resolves itself as your reputation improves. If it doesn't, or if it's getting worse, that's a sign there's something else going on.
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