All about Email Throttling

 In Email Infrastructure

What is email throttling?

Email throttling in simple terms is Internet Service Providers (ISPs) limiting/controlling the intake of emails from a sender if it crosses a certain threshold. You may be blocked by an ISP when you send too many emails at one time.

How do I know that my emails are getting throttled?

Many of the ISPs send a detailed bounce message to the sender. The message will say that the sending frequency/time is 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 the sending time. If you slow down large campaigns, it will help to prevent batches of email from being tagged as spam. Throttling by the ESP is a good practice to make sure that all your emails are delivered safely to your user’s inbox. This helps you maintain your sender reputation.

Most throttling is temporary and the possible reasons include sending too many emails to a receiver/receiving domain or multiple recipients have marked the email as spam. The ISPs temporarily refuse to send more emails, until they see how the existing recipients are responding. If the response seems positive, you can expect the throttle to be removed for your next batch of emails.

How email throttle is different from deferrals?

A deferred email has not been delivered to the sender due to a temporary rejection by the ISP. The delivery attempt is skipped, and it will be retried later.

But they promised that they'll deliver a million emails immediately..

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

The truth is, it’s not the ESP who ultimately decides to limit emails. The ISP's throttle emails. They set the limits.

Best practices

  • Don't overlook email throttling.

Simply adjust the time/frequency for high volume messages, particularly to single domains (like Gmail). Pay attention to time of day, and make sure everyone is aware that it will take time to send out messages, especially in high volume sends.

  • Shun Bounces

You should monitor the percentages of hard and soft bounces. You need an active database to send emails, so that customers click and open emails. The higher the engagement in terms of opens, 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 volume.

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


  • Follow the warm-up process

You should gradually increase the volume of emails you 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. The better the reputation, the higher the chances of your emails landing in your customers inbox.

  • Clean up your email database

It doesn’t matter how good your IP and domain reputation is, since it only takes a handful of emails addresses hitting spam traps or customers marking emails as spam to ruin it. You need a clean email database.

It is essential to clean your database and remove all of the non-existing, inactive emails and unsubscribes. Keep your users engaged in your newsletters, just to keep your database active and fresh. Segment inactive users, and put them into re-engagement campaigns to try and get them back. If they still don't open, mark them inactive and move on.


It is very tough to find out the directives of ISPs to avoid the throttling, as they vary from ISP to ISP. You should follow best practices and maintain good email sending..

Suggested: Check your IP/domain blacklist status with our free blacklist checker. A clean sending IP/domain with automatically improve your inboxing rate.


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