Advanced Email Throttling: Three Essential Tools for Sending Success

Ready for the spotlight?

Even though I’ve been playing for several years now, I’m still pretty much a novice whose focus is on learning the basics. I do sometimes dream about becoming a legendary banjo player, but in reality, I know it would take a lot to make it to the next level as a professional musician. More practice, more exposure within the music industry, maybe a few new banjos…er, tools for the business. And then there’s talking it over with my wife…

In my last post, I gave you the big-picture basics of email throttling—to continue the banjo metaphor, I showed you how to strum along with some simple chords. But today, I’m going to show you how to master throttling using a world-class instrument—GreenArrow. It can be a bit tricky to get the hang of, but once we’re done, you’ll have the skills you need to be a virtual email throttling superstar.

Verse 1: Throttle Templates To The Rescue

The first refrain you need to learn is how to manage Throttle Templates. A Throttle Template is a shortcut for deploying or modifying a set of rules that will apply to any IP addresses on a server using that template. They can not only help you save time, but also make it easier to fine-tune your throttle settings on a case-by-case basis.

For example, because many of our customers initially start sending email from a new IP address, each new installation of GreenArrow is deployed with a default Throttle Template with settings we’ve found to be effective for “cold” IPs. This not only helps customers get started faster but also helps coach them through the IP warm-up process.

GreenArrow Email Throttle Template

Throttle Templates also shine when it comes to managing multiple sets of rules for multiple IP addresses. Let’s say you have eight IP addresses, of which four are already warmed up, and four are new. Instead of manually setting up the same rules on all eight, you can use two Throttle Templates.  One would be used for the warmed-up IPs, and the other for the IPs that need to be warmed-up.

But the best part is when changes need to be made. In GreenArrow, template changes instantly apply to the associated IPs, and if you need to tweak the rules on one IP, you can place an override that only applies to the specific IP address. There is no need to create a whole separate template to adjust a few rules on a single IP temporarily. Music to my ears…

Verse 2: Harmonize Multiple Domains via Domain Groups

Another key to virtuoso throttling is creating Domain Groups. Most of the big ISPs have multiple domains associated with their service; for example,,,,, and others all use the exact same mail servers. For this reason, these domains should be treated together as one unit when sending email.

This is done in GreenArrow by setting Domain Groups:

GreenArrow Email Domain Groups

As you can see, there are three settings available for each Domain Group:

  • Max Concurrent Connections: Total number of open SMTP connections GreenArrow will allow from a single IP address to the Domain Group at any given time.
  • Max Messages per Hour: Rate at which GreenArrow will attempt to deliver emails from a single IP address to the Domain Group.
  • Throttle Program: Automatic throttling program (part of Dynamic Delivery) that will be used for this Domain Group.

All domains listed in a Domain Group are tracked together when applying “Max Concurrent Connections” and “Max Messages per Hour” limits. For example, if and are grouped together and configured to allow for up to 100 concurrent connections, then and may use a combined total of 100 concurrent connections at a time per sending IP. This ensures maximum deliverability by not flooding the same mail servers with too much email at once.

If you have a breakdown of how many subscribers you have at each domain, it’s also a good idea to review the Domain Groups before sending, to make sure any higher volume domains are present. Our documentation on throttling goes into more detail, in case you’re interested.

Chorus: Don’t Be Afraid to Take It Slow

Throttling is a great tool that can not only help build (or maintain) your reputation with the ISPs but also tip you off when there’s an underlying problem. One of the first indications of a possible issue will be the number of messages “In Queue.” Inevitably, there will almost always be some messages in the queue (and even more so initially during the launch. If more than about 10% of the total sending volume for a campaign is waiting in the queue, it could be an indication of a deliverability problem and should be investigated.

However, throttling is not a solution in and of itself. For example, suppose you have a throttle in place that you notice is holding back a large amount of your email. It may be tempting to go to the extreme of using the “No Limit” throttle by leaving the throttle limit value blank. After all, faster is always better, right?


Turning throttling off or increasing it dramatically will increase the number of delivery attempts made to the ISPs (especially with powerful sending software like GreenArrow), but this might not be the right answer. For example, if you have emails in the queue because the ISP is deferring your email due to an underlying deliverability problem, then increasing the throttle limits could hurt your reputation. This is like shouting louder when someone asks you to please be quieter—not the best way to make friends.

It is not possible to know the exact volume at which the ISPs will stop accepting and start pushing back. This is why we recommend a gradual and ongoing cycle of sending a consistent volume of email, monitoring the results from that volume, and adjusting behavior based on those results. This process, while requiring patience, not only develops reputation at a healthy pace, but it does so in a way that allows for adjustment along the way.

It is easier to avoid obstacles when moving slowly, and the reward for taking time with this process is a well-tuned system with throttle configurations that were carefully honed based on data, not guesses.

Verse 3: A Good Throttle Is A Smart Throttle

Once all of the throttling templates are in place, it is time to consider what actions GreenArrow should take in certain deliverability situations. In GreenArrow, this kind of automated throttle program is called Dynamic Delivery.

Dynamic Delivery is a GreenArrow feature that monitors each IP address’ deferral and bounce rates for a Domain Group and automatically reduces throttling settings if it detects issues. This is done by entering Backoff Mode. Backoff Mode automatically ends after the bounce and deferral rates return to normal.

These settings are configured by creating Throttle Programs. Here’s an example of one:

GreenArrow Email Dynamic Delivery

In this case, once triggered, the Backoff Mode would set the throttles on the Domain Group for the impacted IP to:

  • 1 Maximum Concurrent Connection to that Domain Group
  • 20% of the configured Max Messages Per Hour

This particular configuration will be active for 10 minutes (600 seconds) before Dynamic Delivery checks again to see if it should keep this Throttle Program in place.

At the bottom of the configuration page, you can see the delivery parameters and minimum sample size required for Backoff Mode to be triggered. In this case, if more than 100 delivery attempts were made in the past 5 minutes with a failure rate higher than 10% or a combined deferral/failure rate higher than 50%, Backoff Mode will be enabled for the Domain Group using this Throttle Program.

Additional Throttle Programs can be created and assigned separately to different Domain Groups to give exactly the right response when it is needed, all of which is then managed automatically. And if you’d like to add even more automation into the setup, GreenArrow Engine provides APIs for configuring everything we’ve covered so far, like VirtualMTAs, Throttling Templates, Throttle Programs, and more.

Bridge: Mine The Data and Reap The Rewards (or Leave It To Us)

Even though Throttle Programs provide some automation for an immediate response to issues as they arise, monitoring your reputation (and the ISPs responses) requires some level of human attention—even after the IP warm-up has completed. It’s good practice to check-in as part of a regular review of statistics, just as you do when checking subscriber engagement data. This can be as simple as getting in the habit of scrolling through the SMTP Stats looking for anything out of the ordinary, particularly in the Failure, Attempts Deferred, and Attempts Throttled columns.

A number of our customers really enjoy digging into this level of detail and appreciate having access to this depth of control. Other GreenArrow customers choose to focus on their content and subscriber engagement instead of the intricate details of managing throttles and deliverability—they leave that to us. Most new licenses include time with our deliverability team to help you get launched, and we offer a variety of options for ongoing management of your deliverability. So whether you find GreenArrow’s throttles really fascinating or a bit overwhelming, we have you covered!

Outro: Keep On Strummin’ And Pluckin’ Along

While I’m getting better at playing the banjo, for now, I’m keeping my day job. Likewise, while some will find this blog fascinating, I’m sure that many of you have read it and decided that you don’t have the time and energy to “go pro” with your email throttling career. Whatever the case, our team is here to help.

And hey, if you’re managing your own deliverability and/or just want to see these powerful features in action, your free demo is only a click away. Until next time, stay sharp and keep practicing!


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