Limits on email volume are set by email service providers (ESPs). This is a safeguard against spammers who send large numbers of emails all at once. Emails will be deferred due to greylisting if the sender exceeds the acceptable threshold. The mail client will be unable to connect to the server and will receive a 5xx or 4xx error code stating that the user's mailbox has exceeded its quota or something similar. This is known as email throttling.

Definition of email throttling

Depending on which side it is used, this term can have two meanings:

  • Email throttling is the slowing or blocking of email delivery due to an exceeded email delivery rate.
  • Email throttling refers to the practice of intentionally limiting the number of email messages sent through an ESP's server at one time.

If the delivery volume exceeds the established rate, throttling can be performed explicitly by you or by the sending server.

What is the distinction between a throttled email and a deferred email?

An email that has been throttled is one that the sending server has rejected. The delivery attempt was not completed, and the mail client must retry.

A deferred email is one that the receiving server rejects. The delivery attempt was made, but the email was deferred due to greylisting by the recipient server. As a result, the sending server will attempt to send the email again later.

When an email is relayed from the sending server to the receiving server, it can no longer be throttled. However, it is possible to postpone it. That is the distinction. However, some ESPs do not distinguish between these concepts and instead use the term throttling to describe the rejection by the sending or receiving server.

Common causes of throttled emails

Only if you exceed the acceptable limit of email sending during a specified period the emails will be deferred due to greylisting services for your email campaign.

When you attempt to deliver an unexpectedly large number of emails, a red flag is raised. If you do this from a new or even virgin IP address, it will appear even more suspicious. It has no sender reputation and begins to send an excessive number of emails. This appears to be a spammer, and email throttling is the apparent result.

What about abuse reports, full recipient mailboxes, and other similar reasons that circulate on the internet? These are mostly related to deferred emails because they come from the recipient server. Let's take a look at the most common of these.

Motives for delaying emails

  • The mailbox of the addressee is full.
  • The ports of the receiver server are closed.
  • Your IP address is not recognized by the recipient server (warmup is needed).
  • Previous emails sent from your IP received complaints of abuse or were labeled as spam.
  • The bounce rate for emails previously sent from your IP is high.
  • Outdated or invalid email address
  • Spam filters have been activated by emails.

Best practices to avoid having your emails throttled or deferred

No email throttling is expected if you do not exceed the ESP's sending quota. However, you are unlikely to be willing to handle your email campaign with 100K recipients by hand. As a result, the following best practices will be extremely beneficial in increasing your performance and avoiding throttling.

How to Avoid Email Throttling

Discover the limitations of your ESP.

To avoid email throttling, you must adhere to the limits set by your ESP. Before you send your email campaign, you should first learn about them.

Spread emails to avoid ESP email throttling

To limit the number of emails you send yourself. This prevents your emails from being bounced and, as a result, speeds up delivery. One method is to segment emails based on their domain. Some domains, such as,, and, use the same mail servers. When sending emails, treat them as a single entity.

Another option is to divide your email database lists into batches for sending. It's also a good idea to keep marketing and transactional email traffic on separate IPs/domains. This will distinguish their sending reputation.

Schedule email throttling

Some MTAs, such as MailerQ, allow you to limit the speed at which emails are sent. This allows you to set the delivery threshold in terms of bytes or messages per minute, new SMTP connections per minute, and so on. An email throttle schedule can be set up based on the selected limits. It's an interesting feature that can be used to warm up IPs/domains as well as control the entire delivery process. In addition to the throttling schedule, you can take advantage of flood and response patterns:

  • Flood patterns are rules for sending errors from the recipient server. The MTA will pause or throttle email sending to this destination if it is blocked or greylisted.

  • Response patterns send rules that monitor bounce responses by class (500 No such user, 501 Bad address, 550 Invalid recipients) and take appropriate action.

Avoiding email postponements

Use a dedicated IP address.

Most email service providers provide a dedicated IP address as an optional feature. The primary one is a lack of sharing. If an IP address is dedicated, you are solely responsible for its sending reputation.

IP address Warm-up

Email providers and recipient systems look for domains and IP addresses that have previously sent emails. If there is no IP or domain, it is considered cold. Sending a large number of emails from it will appear suspicious and will almost certainly result in a blockage or email throttling. That is why you must first warm it up before gradually increasing the number of emails you send. This practice allows for the establishment of a good sender reputation for new IP addresses and domains.

Improve the sender's reputation

After the warmup, you can use your IP address to its full potential. However, the process of establishing a sender's reputation is ongoing and will never be completed. Each email sent from your IP address is significant. Your reputation will suffer if a few emails end up in the spam folder or, worse, are marked as spam by the recipient. That is why, before you send an email, you should use spam checking software.

Organize your email database

This is yet another technique for improving your sending reputation. Any invalid or out-of-date email addresses should be removed from the email database. Inactive recipients who never open your emails should also be excluded. You can also use them for re-engagement campaigns if you have any.

All of the practices listed above can help you deal with email throttling.


In the event of a deferral, SendGrid keeps delivery attempts for 72 hours. However, they mostly advise keeping an eye on your sending reputation and gradually increasing the volume of emails you send.


MailSlurp sends out your email campaign from a variety of IP addresses. This helps to spread the delivery and avoids email throttling on the recipient's end. MailSlurp claims that any deferral will be handled automatically on their servers.


Elastic Email isn't as tenacious as SendGrid, but it will try to deliver your email for up to 48 hours. It will then be removed from the queue. Elastic Email alerts users when throttling threats are detected. At the top of the screen, an alert appears.


ActiveCampaign offers cool email marketing software. It enables you to configure multiple SMTP connections for sending. Users can set the maximum sending speed and email volume for each connection to control email throttling on their end.

SES from Amazon

In terms of throttling, if the maximum number of emails sent per second is reached, Amazon SES will drop the email and will not attempt to redeliver it. The mail client will also display the error 454 Throttling failure: Maximum sending rate exceeded. This error is recoverable, so you can wait a few moments and try again later.

Conclusion: throttle independently or be throttled by the ESP.

When dealing with large amounts of email, email throttling is unavoidable. However, it is not required to be throttled by the email service provider or delayed by the recipient server. Throttling best practices and a developed sender reputation will elevate email sending to a higher level.