Email Throttling Basics: What You Need to Know

One of my favorite hobbies is practicing the banjo.

And while my wife is a good sport about it, there’s only so much she can take. If I am down in the basement where she can’t hear me, I can play until my fingers fall off. But if she is within earshot of where I’m practicing, I only get to play for a short time before she flat-out tells me to stop.

My wife’s taste for banjo music is a lot like Internet Service Providers’ (ISPs) taste for email. After a certain point in the sending process, certain ISPs may stop accepting emails. And just like with my wife, it doesn’t make sense to continue with delivery attempts because the ISPs will just get more and more annoyed. You can think of throttling, at least in GreenArrow, as our way of preventing you from annoying the ISPs (and having to sleep on the proverbial couch as a result).

Over the past few months, we’ve written a few blog posts on how fast GreenArrow can send email. And while we’ve referenced throttling a few times, we haven’t given the topic a deep dive until now. Today, we’ll cover Throttling Basics, and my next post will cover Advanced Throttling. Here I’ll provide a foundation and big picture for those who need a general understanding of how it works, why you may need throttles, and common pitfalls to avoid. Even if you consider yourself an advanced user, read both as one will build on the other. Let’s get started.

With Great Speed Comes Great Responsibility

To understand what throttling is, let’s look at how it differs from deferrals.

When an email is deferred, it means that a delivery attempt has been made, but the ISP has temporarily rejected it and asked that it be sent later.

Throttling is similar but occurs proactively in GreenArrow. When throttled, the delivery attempt is prevented from happening, and the message will be tried again later. Since we don’t contact the ISP, they can’t be bothered by it. The throttled emails are, in effect, strumming away in the basement until they can come upstairs to play.

Here’s a simplified look at the process:

Email Throttling Lifecycle of an Email Allow me to walk you through it:

  1. The email is created, either by GreenArrow Studio or another application.
  2. Right away, Engine tries to deliver the email.
  3. From that first delivery attempt, one of four things can happen:
    • Success: Your email was accepted by the recipient ISP.
    • Bounce: The email delivery flat-out failed. This is also known as a failure.
    • Deferral: The recipient ISP did not accept the email, but it instructed GreenArrow to try again later.
    • Throttle: GreenArrow held the email back on this delivery attempt, preventing the ISP from seeing there was an email to deliver.
  4. If the email was either Deferred or Throttled, it is saved to be retried later. However, if it has been too long since the email was created (by default two days), then it is considered “expired” and is bounced.
  5. The cycle continues until a Success or Bounce is achieved.

You can see from the chart how similar deferrals and throttling are. The only difference is that throttling is an intentional slowing down (so as to not annoy the ISPs), whereas deferrals often happen when the ISPs have had enough and are asking you to stop.

GreenArrow supports two throttle settings:

  • Max Concurrent Connections
    This is the maximum number of simultaneous delivery attempts that can happen at once. This is like the number of lanes on a highway.
  • Max Messages per Hour
    This is the maximum speed that delivery attempts will be performed at. Kind of like a speed limit for email.

These work together to keep your email sending from being too “loud” to the ISPs.

Avoid the Pitfalls: “You Gotta Know When to Hold ′Em”

“But I just bought this really fast new email software. Why would I want to slow down my sending?”

We can’t say it enough: Being polite to the ISPs (or your significant other) is the key to a great relationship and quality results. Here are three pitfalls you can avoid with your new Throttling know-how.

1) Warm-Up Your IPs Consistently

One of the most common uses for throttling is its role in your IP warm-up strategy. Most high volume senders have a large enough subscriber base that when they have brand new “cold” IP addresses, they cannot simply start sending the full volume of mail without first building their reputation as a sender. This process of building a reputation is called the IP warm-up, and GreenArrow’s throttles can help create the correct, consistent rate at which the ISPs receive emails from your new IPs.

Ignoring limits when warming up a new IP can lead to the ISPs effectively slamming the door in your face. We’ve seen senders get blocked within 10 seconds because the “Max Concurrent Connections” limit was not configured appropriately.

2) Avoid Awkward Moments

Second, throttles make you look good by avoiding any unpleasant interactions—they’re the email equivalent of being sensitive and tactful. GreenArrow throttles to stay inside of the limits that were set — limits that help avoid risking annoying the ISPs.

3) Protect Your Reputation

Finally, another use for email throttles is to protect us from ourselves—and our sending patterns. Consistency is the key to a good reputation, and a sudden major change in your normal volume can hurt your IP’s reputation. Imagine putting that banjo 2 inches from your spouse’s ear and then strumming as hard as you can, and you can see why a sudden spike in volume is usually perceived as a bad thing.

Check Your Throttle Before You Break the Bottle

Here’s another kicker: creating mail faster than what your throttle allows can also cause a problem. Think of the throttle as the small neck of a funnel. If you’re really good, you can pour fluid into the funnel at the same rate as what will be allowed through the neck. Pour a little faster, and it will just build up a bit in the funnel’s reservoir. But there is a point where the reservoir can’t hold anymore, and unless you slow down the rate at which you’re pouring, you’ll have a real mess on your hands.

There was a small but important step in the flow chart labeled “Retry Queue.” This is basically where the throttled emails are being put in a reservoir where they wait to be retried later. Excessive throttling adds a lot of system load in writing messages to this queue, and in extreme cases, it’s possible to completely fill the disk with queued messages. The emails will fail if they sit in the queue for too long, so there is not much benefit in creating a bunch of emails that will sit in the queue being throttled until they expire.

What we hope to achieve is the right throttles for the volume of mail you’re sending or planning to send. Getting it just right is complex and requires precise changes. With GreenArrow’s throttles, we can either adjust the throttle itself (the neck of the funnel) to allow more out, increase the disk (reservoir) size to hold more emails in the queue or reduce the sending volume (the rate at which we are pouring the…um…emails).

Need Help or Have More Questions? We Get It.

Sound complex? Then you’re right on track because this is complex stuff. Even basic email throttling can have big impacts, and it can sometimes take days or weeks for the impact of your changes to show up. This is a big reason why we provide Launch Assistance with most licenses of GreenArrow. We know that the ins and outs of throttling are new to most senders, and even advanced users tell us they appreciate guidance on implementation.

Stay tuned for Part 2: Advanced Email Throttling, where we’ll cover usage, implementation, common management scenarios, settings, and some advanced tools like Dynamic Delivery. In the meantime, want to see it in action? Reach out to our team to schedule a live demo where we can chat about your unique volume, sending patterns, and various email challenges.


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