Email throttling is a common practice employed by the ISP (Internet Service Provider). Funny name heh? You can learn more email marketing terms in our useful glossary. In layman’s terms, throttling means limiting the amount of emails you can send to a particular domain during a specified period of time. Throttling is carried out by the sending server, according to the number of messages accepted per connection by the receiving server.

So why do ISPs define thresholds that must be respected? To simply stop high volume spammers.

The rules and the number of emails are determined by the ISP. Some ISPs are very secretive about their procedures and numbers to avoid suave spammers from exploiting them – while others are more transparent and open to sharing their procedures. Let’s take a closer look at a few:

AOL Mail:

Allows for 500 messages per established connection.

Yahoo Mail:

Allows for 20 messages per established connection. However, the number of connections are solely based on your reputation as a sender.


Number of messages and connections are solely based on your reputation as a sender.

If the above procedures are not carried out and not followed correctly, then the ISP will reject your connection.  If your connections continuously fail or are rejected then the ISP may temporarily block you. This will result in a bounce back.

Reading the above, you may think throttling is quite restrictive and somewhat limiting your email success. However, this doesn’t have to be the case and we can learn and use throttling as an opportunity instead:

  1. Targeted Emails – yes, this may involve some work at first as you sort out your data and top engagers. However, your hard work will pay off in the longer term.  Targeted emails generate more engagement and less unsubscribes, complaints and bounces. In return this will benefit your reputation as a sender and help you avoid wasting valuable email allowances on non-engagers and email accounts that are no longer valid.
  2. Managing Responses – throttling means the delivery of your emails are spread out which can help you manage your responses and feedback better. This can be recipients replying back to your emails, events or calling your helplines. Additionally, this can help you manage traffic to your website too. At Campaignmaster, our deliverability team are constantly working directly with ISPs to make sure your emails are delivered. We achieve this by following the rules and procedures put in place by the ISPs. We provide all our clients with a warmed up IP address so you have a great sending reputation from the start.

Having a warm and trusted IP allows for more connections and in return messages delivered.

It’s important to maintain a good sender reputation. This can be achieved with good data management, ensuring your lists are up to date, conducting re-engagement programmes and sending targeted campaigns. This will help reduce bounces, complaints and unsubscribes and will lead to a better sending reputation.

Campaignmaster provides clients with even more control, by allowing them to request throttling on their account. Enabling throttling on your account allows you to limit the number of emails sent out per hour, minute and second even.

Actually, we go even further, sending a high volume of emails to a certain domain or organisation? We can even apply throttling for particular domains.

Get in touch with us at or call us directly on + (0) 44 208 863 5334 for email marketing advice and support.