All about Email Throttling

All about Email Throttling

Posted under Email Best Practices , Email Delivery on October 26, 2017

What is email throttling?

Email throttling in simple terms is an 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. These errors are most often 400 errors – indicating a temporary failure.

Here are some of the common reasons for throttling-

  • The recipient’s mailbox is full
  • The receiving servers are having bandwidth issues or do not have open ports
  • Your IP address might not be known by the receiving server and is afraid you are sending spam emails

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 a 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 the throttling is temporary and the possible reasons could be 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..

Yes, 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.

Here are the best practices to avoid Email Throttling:

  • Email Throttling is often overlooked – Simply adjusting the time/frequency with which the messages are sent and spreading out the emails being sent instead of sending a huge volume at once, particularly to a family of domain can help with avoiding email throttling when it comes to email delivery.
  • Shun Bounces – While sending out emails, it is recommended to maintain a ratio of hard and soft bounces. It is always good to update your database with active email addresses to send out emails to, so that you get a good click and open rates. Higher the engagement in terms of open rates, link clicks, CTR, the better the reputation.
  • Separate your marketing and transactional volume – It’s better to use separate sending domains and IPs for marketing and transactional emails. Transactional emails are emails that the receiver expects and is looking forward to. But marketing emails are not as welcome in the inbox. So it is always better to not lower the sender reputation when it comes to the space used for sending transactional emails. Not convinced? This is why you need a dedicated IP for sending a high volume of emails.
  • Follow the warm up process – Warm-up process is when you keep the sending limit lower and gradually increase it with time. This is the best practice when you are sending a high volume of emails. May it be a dedicated IP or a shared one, it is very important to follow the warm-up process to maintain a good reputation with the ISPs. Better the sender reputation, higher the chances of your emails landing in the recipient inbox, and who doesn’t expect that.
  • Clean up your email database – If your email database is not clean, it doesn’t matter how good your IP’s and domain’s reputation is, only hand full of emails to that list will destroy it. It will be perceived as unsolicited mailing by the sender. Hence, it is essential to ensure that you clean your database and remove all the non-existing, inactive emails and unsubscribed email addresses. 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.

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Akshay Tiwari|Inbound Specialist

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