Manage Email Throttling Right!

On October 28, 2019
6min read
Stanislav Shymanskyi Product Manager @Mailtrap

Email service providers (ESPs) set up limits on the email volume you can send. This is a measure to protect against spammers that tend to send big amounts of emails at once. If a sender exceeds the acceptable threshold, the emails will be bounced back. The mail client won’t be able to connect to the server and will get a 5xx or 4xx error code claiming that the user’s mailbox is over quota or something like that. This is called email throttling, and in this article, you’ll learn how to use this for your own benefit.

Email throttling definition

This term may have two definitions depending on the side it’s implemented on:

  • For email services: email throttling is slowing-down or blockage of mail sending due to an exceeded email delivery rate.
  • For users of email services: email throttling means intentional limiting the amount of email messages sent through an ESP’s server at one time.

So, throttling can be carried out explicitly by you or by the sending server, if your delivery volume exceeded the established rate. But there are also other reasons that we’re going to explore a bit later.

What is the difference between a throttled and deferred email?

A throttled email is an email rejected by the sending server. The delivery attempt has not been carried out and the mail client needs to retry it. 

A deferred email is an email rejected by the receiving server. The delivery attempt has occurred, but the email has not been accepted by the recipient server. So, the sending server will try to send the email later. 

When an email has been relayed from the sending server to the receiving one, it can not be throttled any more. But it can be deferred. And that’s the difference. However, some ESPs do not separate these notions and use the term throttling to define the rejection by sending or receiving server. 

Typical reasons for throttled emails

Email services will throttle your email campaign for only one reason – if you exceed the acceptable limit of email sending during a specified period.

 The red flag is raised as soon as you try to deliver a suddenly increased flow of emails. It will look even more suspicious if you do this from a fresh or even virgin IP address. It has no sender reputation and begins to send too many emails. This looks like a spammer, doesn’t it? In this case, email throttling is quite an obvious result. 

And what about abuse reports, full recipient mailbox, and other similar reasons that are kicked around on the web? Those mostly relate to deferred emails since they originate from the recipient server. Let’s check out the most common of these.

Reasons for deferred emails

  • The recipient mailbox is full.
  • The recipient server’s ports are inaccessible.
  • The recipient server doesn’t recognize your IP address (warmup is needed).
  • Emails previously sent from your IP had a negative response – got abuse reports or were marked as spam.
  • Emails previously sent from your IP have a high bounce rate.
  • Invalid or outdated email address.
  • Emails have triggered spam filters. For more on this, read Why Emails Going to Spam and How to Prevent It

Best practices to avoid your emails being throttled or deferred

If you don’t exceed the ESP’s sending quota, no email throttling is expected, right? But you are unlikely to be willing to deal with your email campaign for 100K recipients by hand. That’s why the following best practices will be quite useful to increase your performance and avoid throttling. 

Avoiding email throttling

Learn your ESP’s limits

To prevent your emails from throttling, you need to meet the limits set by your ESP. Learn them first before sending your email campaign. 

Spread emails to avoid email throttling by the ESP

You can control the volume of emails for sending yourself. Why? By doing this, you prevent your emails from being bounced back and, hence, speed up the delivery. One way is to segment emails by domain. Some domains use the same mail servers, for example,,,, and so on. Treat them together as one unit when sending emails. 

Another way is to split your email database lists into several batches for sending. Also, it is a good practice to use separate IPs/domains for marketing and transactional email traffic. This will separate their sending reputation. 

Use email throttling schedules

Some MTAs, for example, MailerQ, provide capabilities to limit the sending speed of your emails. You can configure the delivery threshold by bytes or messages per minute, new SMTP connections per minute, and so on. Based on the chosen limits, you can set up an email throttle schedule. It’s a cool feature that you can use for warming up IPs/domains, as well as to control the whole delivery process. In addition to the throttle schedule, you can benefit from such functionalities as flood and response patterns:

  • Flood patterns are sending rules for errors from the recipient server. In the case of blockage or greylisting, the MTA will pause or throttle email sending to this destination.
  • Response patterns are sending rules that track bounce responses by class (500 No such user, 501 Bad address, 550 Invalid recipient) and initiate a suitable follow-up action.

Avoiding email deferrals

Use a dedicated IP

Most email service providers offer a dedicated IP as an optional feature. What benefits can you expect? The major one is the lack of sharing. If an IP is dedicated, only you bear responsibility for its sending reputation. 

Warm up the IP

Email providers and recipient systems check domains and IPs for previous sending experience. If your IP or domain has none, it is considered cold. Sending large amounts of emails from it will look suspicious and will absolutely cause blockage or email throttling. That’s why you need to warm up it first – gradually increase the amount of email you send. This practice allows you to establish a good sender reputation for your new IP and domain.

Pump up your sender reputation

After the warmup, you can use your IP address at full capacity. But the process of building a sender reputation is not over and will never end. Each email sent from your IP matters. If a few emails hit the spam folder or, even worse, were marked as spam by the recipient, your reputation will drop. That’s why you should not neglect spam checking tools before you send your mail. We introduced some reputable ones in the Email Testing Checklist

Clean up your email database

This is another practice aimed at strengthening your sending reputation. Your email database should be free of any invalid or out-of-date email addresses. Also, exclude inactive recipients – those who never open your emails. Alternatively, you can use them for re-engagement campaigns if any.  

All the aforementioned practices are what you can do to cope with email throttling. And do ESPs keep track of this somehow? Let’s check out below.

How the popular email sending services handle throttling


SendGrid keeps delivery attempts for 72 hours in the case of deferral. But mostly, they recommend watching your sending reputation and gradually increase the volume of emails to send.


MailChimp uses email throttling for their own benefit. They send your email campaign from multiple IP addresses. This spreads the delivery and helps avoid email throttling from the recipient’s side. In the case of any deferral, MailChimp claims to handle this automatically on their servers.


Elastic Email is not as persistent as SendGrid – they will attempt to deliver your email for up to 48 hours. After that, it will be removed from the queue. Elastic Email warns their users in the case of throttling threat. The warning pops up at the top of your screen. However, if the issue has occurred, you have to deal with it on your own or contact support.


ActiveCampaign provides a cool software for email marketing. It allows you to add multiple SMTP connections for sending. Users can configure the maximum sending speed, as well as the email volume, for each connection. So, you can control email throttling at your side, which is awesome!

Amazon SES

If you still don’t know What Amazon SES Is And How to Use It, you are welcome to read our blog post. As for throttling, if you reach the maximum number of emails for sending per second, Amazon SES will drop the email and won’t attempt to redeliver it. Also, your mail client will get an error 454 Throttling failure: Maximum sending rate exceeded. This error is retriable, so you can wait a bit and try to send the email at a later time.

Conclusion – throttle by yourself or you’ll be throttled by the ESP

What is the key point of the article? Email throttling is inevitable when you deal with sending large amounts of emails. But it’s not necessary to be throttled by the email service provider or deferred by the recipient server. A developed sender reputation and the use of throttling best practices will shift your email sending on a higher level. Do your best and good luck!

Article by Stanislav Shymanskyi Product Manager @Mailtrap