Want to know All about E-Mail Throttling?

All about Email Throttling

October 26, 2017

What is email throttling?

Email throttling in simple terms is ISP limiting/controlling the intake of emails from a sender if it crosses a certain number. It is highly possible that you’re blocked by an ISP for sending a high volume of emails at a time.

How do I know that my emails are getting throttled?

Many of the ISPs throw back a detailed bounce message to the sender regarding the sending frequency/time being too quick.

Why it is important?

ISPs react differently to receiving large volume of emails from one common source. Some of the ISPs levy permanent or temporary sending volume restrictions.

Most of the ESPs prefer controlling down the sending time. Slowing or throttling down mass campaign helps to differentiate the authentic batch of email from getting tagged as spammy content. Throttling is a good practice to make sure that all your emails are delivered safely to your user’s inbox helping to maintain your sender reputation.

Most of these throttling are temporary and the possible reasons are either you’re sending too many emails to a receiver/receiving domain or some of the recipients have started marking the email as spam. Hence, the ISPs temporarily start refusing to send more emails, until they see how the existing recipients are responding. If the response seems positive, you can expect the throttle to get removed soon for your next batch of emails.


How email throttle is different from deferrals?

A deferred email is an email which was not delivered to the sender as it was temporarily rejected by the ISP and asked to be sent later.

Whereas in case of a throttled email, the delivery attempt is skipped and a retry attempt for sending the message is made.

But they promised that they’ll deliver a million emails in a go..

True, every ESP says this when you sign up for their service.

However the bitter truth is, it’s not the ESP who decides on limiting the emails. Throttling is done on the ISP’s side which sets a limit to the sending volume.

On the other hand, ESPs can’t go against the ISPs, they have to maintain a superior relation to maintain the inbox quality.


Best practices

  • Email Throttling is often overlooked when it comes email delivery, due to prioritizing other factors over throttling. Simply adjusting the time/frequency with which the messages are sent with huge volume, particularly to a family of domain.
  • Shun Bounces

While sending out emails, it is recommended to maintain a ratio of hard and soft bounces. It is always good to nominate an active database to send out emails to, so that you get good click and open rates. Higher the engagement in terms of opens, clicks, CTR, better the reputation.

  • Separate your marketing and transactional volume

It’s better to use separate sending domains and IPs for marketing and transactional volume.

Not convinced? This is why you need a dedicated IP for your separate volumes.


  • Follow the warm up process

Warm up process is the best practice to gradually increase your volume of emails your send. May it be a dedicated IP or a shared one, it is very important to follow the warm up process so as to maintain a good reputation with the ISPs. Better the reputation, higher the chances of your emails landing in inbox, and who doesn’t want that.

  • Clean up your email database

If your email database is not clean, it doesn’t matter how neat your IP’s and domain’s reputation is, only hand full of emails to that list will destroy it.

Hence it is essential to ensure that you clean your database and remove all the non-existing, inactive emails and unsubscribes. Keep your users engaged in your newsletters, just to keep your database active and fresh.



It is very tough to find out the directives of ISPs to avoid the throttling, directives vary from ISP to ISP. The only way to stay away from it is to follow the best practices and get yourself recognized as a good email sender by the ISPs. Post this you may not face deferrals or throttling.

Found this blog useful? Please rate us
1 Star2 Stars3 Stars4 Stars5 Stars (1 votes, average: 5.00 out of 5)

Write a comment


Akshay Tiwari|Inbound Specialist