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Mail Sending Throttling Settings Explained

The downloaded version of Active Campaign Email Marketing (which is installed on one’s own web server) allows users to add multiple SMTP connections for sending. The software will rotate between each SMTP sending connection available — this allows users to spread out the sending load among multiple SMTP servers, which can offer considerable benefits with regards to performance and deliverability.

In addition to offering the ability to add multiple sending connections, our software allows users to configure the maximum sending speed for each connection, and also the number of emails to send with each connection before moving to the next. Since many server hosting companies impose hourly or daily limits to mail sending, and many ISPs score a server’s sender reputation in part based on the number of emails sent in a given time frame, this is an important feature of our software to understand. This article explains how to set up multiple email connections, and how to configure the sending speed and “emails per cycle” settings to meet your needs.

Adding An Email Connection

To add a new email sending connection in Email Marketing 5.0.X and 5.1.X, click on the “Mail Settings” option under the Settings menu. In Email Marketing version 5.2.X and above, click on the Settings button in the upper right corner of the page. (This article will use images from version 5.2.)

Click the option that says “Add a new mail sending connection” to add a new connection. In most cases, you will want to choose “SMTP” as the sending type, and then enter in your SMTP server’s connection details accordingly. You can also add a connection name, choose throttling settings, and choose which user groups can use the connection to send. The image above shows an example where there two connections set up on the Settings page.

Enabling Throttling

To view throttling settings, add a new connection or edit an existing one. There will be a section on the page called “Sending Speed,” with three options: Send without limitations (default), Enable sending throttling and pausing, and Limit number of emails to send for a specific time period. 

Send Without Limitations 

 This setting will send as quickly as possible on your server, without any throttling.

Enable Sending Throttling and Pausing

This option will allow you to choose the number of seconds to pause between sending a certain amount of emails. For example: “Your sending will pause for X seconds after Y emails are sent.” This means that the software will send Y number of emails as fast as possible, then pause for X seconds, then send Y emails again, then pause for X seconds again, etc.

The above settings in the image would send 50 emails as quickly as possible on the server, then pause for 5 seconds. After pausing for 5 seconds, another 50 emails would be sent out as quickly as possible, and so on. This would work out to about 100 emails per 1-2 minutes.  Actual speed would ultimately depend on the speed of your server & mail server though.

Limit Number of Emails To Send Per Time Period

This option allows you to enter in a limit of a certain number of emails to send either per day or per hour. The software will figure out how many seconds (or fractions of a second) to pause between each email that is sent, so that the correct number of emails per hour or per day are sent.

In this example (500 emails per hour), 1 email would be sent about every 7.2 seconds. That works out to approximately 500 emails per hour. (There are 3,600 seconds in an hour, and if you divide 3,600 by 7.2 seconds, you get 500 emails per hour.)

Automatic Calculation

Each time you change the sending speed for a sending connection, the “estimated sending speed” will change automatically on the bottom of the screen, giving you some mathematical estimates on how many emails will be sent per hour, minute, and second. The numbers seen here are purely mathematical and do not take your server hardware or configuration in to account. These numbers represent the maximum sending speed possible using the throttling settings you entered in.

If you do not use throttling, there is no way for our software to estimate your sending speed. Sending speed is determined by your server performance.

Emails Per Cycle

If you have more than one email sending connection set up, you can edit each connection to change the “Emails Per Cycle” setting. (Note: you must have at least two connections set up before you see this option.) Emails per cycle determines how many emails to send with the given connection before moving on to the next connection. By default, it is set to 50 emails per connection. This setting is very useful, as it allows you to distribute more emails with a certain server than with another, if you wanted to do so.

The Emails Per Cycle option is found above the Sending Speed settings:

This image shows a Settings page where there are two sending connections set up, and each has an Emails Per Cycle value of 50:

This means that when you are sending a campaign, Connection 1 will send 50 emails, then the software will switch to Connection 2. Connection 2 will also sent 50 emails, then switch back to Connection 1, and so on.

Advanced: Figuring Out Your Optimal Settings

(This section of the article is for advanced users who want to send as quickly as possible using multiple connections. Please exercise caution and read carefully!)

Many users will purchase a number of SMTP servers from ISPs, but their SMTP servers will have hourly sending limits. Now that you understand how sending speed works, and also how the software rotates between sending connections, you can use that knowledge to determine the best settings for you, so that you can send as quickly as possible, while staying within your sending server limitations.

For example, let us say that you have 5 SMTP servers, and each server can only send 600 emails per hour, but you want to send as quickly as possible using all 5 of these SMTP connections. A common mistake would be to add all five SMTP servers, with throttling set to “600 emails per hour,” and leave the “emails per cycle” at 50. This would not achieve the fastest speeds for you. These settings would send 50 emails with “Connection 1” at a speed of 600 emails per hour (which works out to about 1 email every 6 seconds), then move on to “Connection 2” and send another 50 emails at a rate of 600/hour, then move on to the third connection and send 50 more at a rate of 600/hour, etc. The total overall sending rate in this scenario would still be a maximum of 600 emails per hour, but spread out over all 5 of your SMTP connections.

What you should do if you wanted to send as quickly as possible using all of your connections would be to set the sending throttling settings to 3000 emails per hour, and set the Emails Per Cycle to 600. This would send 600 emails with “Connection 1” at a rate of 3000/hour, which would take about 12 minutes to do. Then it would move on to “Connection 2,” and send another 600 emails within 12 minutes. It would then move on to “Connection 3,” etc. By the time the software has rotated through all five connections, one hour will have passed, and approximately 3000 emails will have been sent out, but none of the SMTP connections will have gone over their limit.

If you have a certain number of SMTP connections available, and they all have the same hourly sending limit, an easy way to find out the fastest you can send is by following this math:

Number of SMTP connections = X
Hourly sending limit for each = Y

(Y / (60 / X)) * 60 = Hourly Sending Speed

Always use Y as your Emails Per Cycle value, and enter the Hourly Sending Speed you figured out above in to your throttling settings. (Note: this method only works if each of your SMTP connections have the same hourly sending limit!)

IMPORTANT! Be extremely careful when configuring your sending throttling speeds!

This method assumes you are sending one campaign at a time only!

This method only works if each of your SMTP connections have the same hourly sending limit!

This method also assumes you have no autoresponders or email confirmations being sent out at the same time as your campaign! To be safe, we usually recommend setting the sending speed a little bit slower than the Hourly Sending Speed that you have calculated, to account for anomalies such as these.

Different ISPs calculate your email sending limits differently! Some may simply cut off your sending once you reach the hourly limit, where others may look at the speed at which you are sending, determine it is too fast, and shut down sending, even before you reach your hourly limit. Make sure to contact your ISP to verify that you will not be breaking their terms of service.

Things To Keep In Mind

  • It is possible to give each mail connection different throttling speeds. Changing the throttling speed for one mail connection will not affect the speed of any other connections.
  • The software will NOT send using all of your SMTP connections at the same time! It will rotate between them, using only one connection at a time.
  • It is usually a good idea to set throttling settings in our software a little bit lower than what your sending limit is on the server. This will account for any other emails that may be generated by other processes on your server at the same time (such as autoresponders, other simultaneous campaigns, individual emails being sent from other software, or from your website, or from users on your server, etc).
  • It is not possible to send any slower than 1 email every 5 minutes. This is because every time your “Sending Engine” scheduled process runs (which is every 5 minutes by default), 1 email will be sent. This is due to the way our sending engine was designed.
  • The “Emails Per Cycle” setting will not appear unless you have at least two sending connections set up.
  • The “Estimated Sending Speed” which is calculated for you when you editing a connection’s throttling settings is not exact or guaranteed. It is simply a mathematical limit calculated based on the numbers you typed in for your throttling settings. It is the theoretical maximum sending speed possible. Your actual sending speed might be slower depending on server performance.
  • Should you run in to any questions or problems, please do not hesitate to contact our support department for further assistance!