What is message throttling and how to detect it?

Your campaign just went out, and most messages delivered within minutes. But a handful of emails to the same domain are sitting in your queue, retrying every few hours. No hard bounce. No block. Just... slow. That's message throttling at work.

Message throttling is when a receiving mail server deliberately limits how fast it accepts mail from you. It's a form of rate limiting. The server doesn't reject your email outright. It pushes back with a temporary deferral and says, essentially, "slow down and try again."

Receiving providers throttle for a few reasons. They protect their own infrastructure from being overwhelmed. They manage senders with lower reputation scores without cutting them off entirely. And they use throttling as a soft signal that something about your sending pattern looks off.

How to spot throttling in your logs

The clearest sign is a flood of 4xx temporary deferral responses. These are not permanent rejections. They're the receiving server saying "not right now." Common examples you'll see in SMTP logs include codes like "421 Too many connections," "450 Please try again later," or "452 Too many messages." (If you're seeing 5xx codes, that's a different problem entirely.)

Beyond the error codes, watch for these patterns:

• Growing queues to specific domains. Messages pile up faster than they're going out, and it's isolated to one provider like Gmail or Outlook.
• Wildly inconsistent delivery times. Some messages to the same domain arrive in two minutes. Others take four hours. That gap is throttling in action.
• High deferral rates in your ESP dashboard. Most ESPs surface this under delivery metrics. If deferral rates spike for one provider, you're being throttled by that provider.

What to do about it

First, don't panic. Throttling is a warning, not a wall. But don't ignore it either. It often precedes something more serious if the underlying cause isn't addressed.

Reduce your sending speed to the throttling domain. Spread large sends over longer windows rather than blasting everything at once. Then check your sender reputation. Throttling frequently shows up when your reputation has dipped, maybe from a recent campaign that generated complaints, or a list that hadn't been cleaned in a while.

So if throttling persists for days and your reputation looks clean, consider reaching out to the provider's postmaster team directly. Both Google and Microsoft have postmaster portals that give you additional reputation signals you won't find anywhere else.

If you're staring at a log full of deferrals and not sure where to start, our SOS hotline is free and we actually pick up.