Controlling the send rate of a campaign

Minimum number of contacts for controlled sending to work

Controlled sending only works if your campaign is being sent to at least 4,000 contacts. Otherwise your campaign will be sent immediately, using the default speed.


If you're an account holder that emails large amounts of contacts in one go, then you may be interested in being able to control and spread the send rate of your campaigns over an even longer period than our default ‘large send’ speed.

This can help protect your email sending reputation with big email providers. Spreading large sends over a period of time will also help ensure your customer service phone lines and resources aren’t swamped should your campaign include a telephone call-to-action.

This article explains how you can control and spread your campaign's send rate.

Enabling and using controlled sending

Our system automatically throttles the send of high volume campaigns anyway, in order to protect your reputation. This is the default ‘large send’ speed and it's still quick, limited only by compliance with send volumes imposed by the ISPs managing the inboxes you're delivering to. 

However, you may wish to control and spread this send rate over a longer period than the default speed allows.

The feature is disabled by default, and can be turned on by following these steps:

  1. Go to Settings > Account > Account settings.
  2. In Features, check the Enable send throttling check box.
  3. Select Save settings.

When choosing your contacts and scheduling your campaign, the feature will appear in the second step.

Select Spread my campaign send over at least: and choose from a range of time periods to spread the send of your campaign over, from 1 hour through to 24 hours.


The send will be broken down into chunks (of 4000 emails) over the time period you've selected.

There are a couple of important points that need to be noted about making use of this type of send, however:

  • Delayed reporting - Reporting won't be available until the whole send has completed. For example, you'll be unable to access any reports for a whole day if you've chosen a spread of 24 hours.
  • Use with paused/unpaused campaigns - If you're using controlled sending and have paused and then unpaused a campaign, be aware that the assured time of the spread send will no longer apply. This is because the remainder of the unsent emails will be sent immediately upon unpausing.

Alternatively, you can select Optimised to send your campaign using send-time optimisation. This will start sending your campaign immediately but will optimise the time of individual sends to maximise open rates based on your contacts' historical opening behaviour. The campaign will optimise over a period of 24 hours from the initial time of sending and will remain in the outbox until complete.

Send time optimisation isn't available for a split test or scheduled campaigns.

Have more questions? Submit a request


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    This is not really of any use.  The benefit of staggered sending would be to send over a period of days rather than hours.

    The ability to choose to send upto X amount today and the remainder tomorrow or next week would be better to "ensure your customer service phone lines and resources aren’t swamped".  Sending the whole lot over 24 hours will not achieve this.

    For example, one of our clients asks us to send to half their address book today and the other half in 3 days time so they can manage the responses.  To do this, we need to export the whole address book, split it into two and re-import to create the 2 new "half" address books.  Then, send the campaign to book1.  Then duplicate the sent campaign and send it to book2.

    Disappointed in this solution given the feedback provided in this thread.

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    Lee, I think you could get around that by adding a field to the contact list (before importing it) that gives all contacts a unique number (or says which 'half' they are in). Then you can use segmentation to filter down to, for example, unique numbers 1-100  and another segmentation to filter down to unique numbers 101-200 another day. This way, you wouldn't have to export and import each time.

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    This could possibly be useful as our recipients are spread around the world and we don't do many region-specific mail outs..

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    Hi Lee

    There is a simpler solution to splitting the address book in half, as long as you don't mind the split not being truly random and the resulting split not being exactly half. 

    Create a segment using a random data field query such as "surname contains j". Run a count and see how close it is to half of the address book. You can then add more queries such as "surname contains d". Keep re-generating the count and making adjustments until you've got approximately half of the address book in the query.

    This only takes a couple of minutes and is much quicker than exporting and re-importing the whole database.

    Hope that helps.


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    I agree with Lee. We can have a work-around such adding extra data items or splitting address lists, but this is time consuming and it would be much simpler just to be able to throttle over several days.

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    Murray - thanks for your comments.

    It is more complicated than what I originally wrote - there are many address books and one particular address book gets updated from the CRM system prior to the sending of the campaign.  We then export ALL contacts, split into 2 or 3 & send 3 separate campaigns - which also means that the stats are split into 3.

    I would have hoped that once the final number of contacts to send to are determined, you could then choose the number of batches to split it into.  You could then choose whether to split evenly or assign specific numbers to each batch.  Finally, you would be able to schedule the sending of each batch as you can currently with campaigns.

    An opportunity missed I fear....

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    Lee, thanks for the clarification. I think what you're suggesting is a very good idea, so I have raised it as a feature request on the suggestion forum.

    If you have access to that, please go and vote for it, so that we can get this feature prioritised.

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    I have added another feature request of something that bugs me - the test campaign checkboxes moving as you click them:

    Hopefully it will get ticked-up and rectified...

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    Fantastic that this feature has been added now, very valuable to firms that want to spread the load of http requests from customers when sending large volume e-mails.

    Have to say that it's key that this option gets added to the API has a priority for those of us planning to send large campaigns via the API.

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    Thanks to everyone for the feedback on this feature. We can certainly appreciate the reasons for the further suggestions that are being expressed here, so thank you for these.

    The reason why we have initially made it function this way is to ensure it avoids interfering with the ReMail feature. As you’ll appreciate, complications would arise should you set ReMail for one day after your initial send whilst having scheduled a throttled send for more than a day later. You would inadvertently create the scenario of a reminder arriving into an inbox before your subscriber has even received the first email!

    It has also been calibrated this way to fit in around likely customer service hours. We wanted to give you enough control to avoid the possibility of campaigns with a call-to-action arriving into inboxes at a time when your customer services aren’t available. The longer the throttled time period selected, the less control you might have over anticipating this.

    These are the reasons why we have initially launched the feature with a sending spread of over 24 hours.

    This is, of course, the first release of throttling. The next iteration of this feature (whenever that may be) will most likely include a more thorough interaction with ReMail and other features.

    As has been helpfully suggested, there are possible workarounds should it not currently suit your requirements.

    However, if you would strongly like us to develop the feature in the direction Murray requests, with Lee’s input, then please do vote for it so we can gauge demand.

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    Hi Neal, thanks for the update.

    For those of us happy with functionality of spreading over a few hours, are there any plans to integrate this into the API campaign creation and send methods? We send three to five e-mails a day to a large number of recipients and we'd absolutely love this feature.


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    Hi there, with the 'Spread my campaign over <N> hours' functionality, are you able to see a histograph of read rates? This would be a valuable tool for analysing the response rate to certain subject lines.



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    I would also like to know the same thing as Ben. I've used the 'spread my campaign over X hours' function for the first time, but would like to get some stats on the open rates over the time of the send 'spread'. i.e. did more people open in hour 1 compared to hour 5?



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    Hi Derrick. I see that you've created a feature request for this - - and I'd encourage anyone who would like to see this functionality to follow the link and vote for it. That way, we'll be able to gauge just how much support there is for it and, if it's a lot, we can make it part of our future development plans.

    In the meantime, you could try clicking on the graph icon next to 'Engagement' on the campaign's report overview screen, and then select the 'First 24h' tab. This gives a breakdown of the amount of opens per hour for the first 24 hours. However, note that the figures given are for total opens, and not unique opens.

    Edited by Neal Goldsmith