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Email Throttling 101: Why it Happens and How to Avoid It?

email throttling cover image with a throttle lever device

Email throttling is when internet service providers limit the number of emails sent within a specified timeframe. 

Have you ever crafted a perfect email campaign, sent it to thousands of people, and waited for a response — and instead, you got the dreaded “you have reached your daily sending quota.”

This phenomenon is called email throttling and can happen for different reasons. To help your emails land squarely in recipients’ inboxes, we’ll go over the critical aspects of email throttling.



What is Email Throttling?

Email throttling refers to the practice of limiting the emails sent from a particular sender or email domain for a specific time period. Internet Service Providers (ISPs) do this to control the flow of emails and prevent spammers from clogging people’s inboxes.

Imagine you have a huge email list and want to send an email campaign to all subscribers simultaneously. The increase in the number of emails can trigger an ISP’s throttling mechanism. 

Email providers may then slow down the delivery of emails, delay them, or completely block them. They do this to protect their users from falling bait to spam and reduce inbox overload.

But for legitimate senders like you, this can be a frustrating experience. That’s why we’ll discuss the major reasons for the throttling issue and help you fix them if you encounter it.


Why Does Email Throttling Happen?

The ISPs have a pretty tough job — they need to prevent spam emails while letting legitimate emails flow through. Email throttling is their biggest weapon in this fight. 

Email providers set specific limits for each sender and can throttle emails for the following reasons:

  1. A sudden spike in email sending: A sudden increase in email sends can overwhelm ISP servers and trigger throttling. 
  2. Low engagement rates: If your emails consistently get deleted or marked as spam, ISPs may consider you a low-quality sender. Low email open and click-through rates can lead to ISPs prioritizing emails with higher engagement rates.
  3. Using a new IP to send emails: If you’ve been sending too many emails from a new IP, email providers may initiate IP throttling.
  4. Poor sender reputation: When some of the email recipients start marking your emails as spam, your sender reputation score takes a hit. ISPs are more likely to throttle emails from senders they don’t trust.
  5. Unkept email lists: Consistently sending emails to inactive or invalid email addresses or spam traps can cause email providers to put a hold on your emails.
  6. Lack of authentication protocols: If your emails lack proper email authentication protocols such as SPF, DKIM, and DMARC, ISPs may begin to suspect your legitimacy and eventually throttle your emails.

During any of the above instances, the ISP will send you an error message to keep you updated about your email delivery.


Regular Email Throttling Error Messages 

You may receive one of the following throttling error messages based on your circumstances:

  • User’s mailbox is over quota,
  • User is receiving mail at too great a rate right now, please try again later,
  • Delivery failed,
  • Daily message quota exceeded.

While not all messages indicate throttling issues, sending too many emails suddenly can risk prompt email delivery.

ISPs usually favor clean senders, but not all providers handle throttling in the same manner. Some have higher email-sending limits than others. 


Common ISPs and Their Throttling Limits

Here’s a quick overview of the well-known ISPs and their sending limits that determine email throttling.

Email ProviderEmail Send Limit
Gmail Around 500 emails per day or 20 outgoing emails per hour
Yahoo Varies based on your sender reputation
Outlook300 emails per day 
AOL mail500 emails per day 
Common ISPs’ Email sending limit

This is a gist of the ISPs’ sending limits, which may change with your sender reputation and the recipients’ engagement rate with your emails.

When you exceed the send limit, you may receive errors related to throttling or deferred email. We’ll review their differences further down. First, let’s understand how to prevent email throttling for better deliverability. 


How to Avoid Email Throttling: 7 Actionable Tips

Although you rely on your ESP to maintain sender reputation and deliverability rate, it’s essential to undertake remedial measures to prevent throttling. 

Here are a few steps you can take to be more aware of email throttling and prevent it:

  1. Space out email sends: Refrain from sending all emails at the same time. Use a scheduling tool to send them in batches. 
  2. Build your sender reputation: New senders with high email volumes are more prone to throttling. Warm up your IP and gradually increase the email frequency. 
  3. Maintain a clean email list: Keep a close watch on invalid email addresses and remove them periodically. A cleaner email list will show ISPs that you’re a legitimate sender. 
  4. Keep content clean: Avoid spammy language and overly promotional content. Focus on clear and concise communication that adds value to recipients. 
  5. Authenticate your emails: Verify your sender identity by implementing email authentication protocols such as DKIM and DMARC.
  6. Segment your email traffic: Segmenting your email traffic into marketing emails and transactional emails can help you send relevant email content and maintain separate traffic.
  7. Use an email automation tool: Use a email marketing tool that sets daily send limits to make your work error-free.

You can prevent throttling and ensure safe sending practices with the above tips. 

However, in unforeseen circumstances, you may receive a deferral message instead of a throttle message. Knowing the differences between deferral and throttle is critical for an enhanced email marketing approach. 


Email Throttling vs. Deferred Email — Understanding the Delivery Hiccups

A deferred message is a temporary delay of an email message by the delivery server. Emails can be deferred due to temporary network issues, recipients’ mailboxes being full, or the recipient’s email server being unavailable. 

In this scenario, the receiving server has received the message but decides to postpone delivery due to issues with the recipient’s inbox. This means your emails will get delivered, just not immediately.

While email throttling and deferrals are used interchangeably, they are different; here’s how.

AspectEmail Throttling Deferral 
Origin Sending serverReceiving server
ActionEmail not sent (Rejected)Email sent, but delivery delayed
ReasonSending limit reached, poor sender reputationTemporary issues, recipient server busy
ImpactImmediate delivery failure Potential delay and eventual delivery 
ResolutionAdjust sending frequency, improve sender reputationWait for the recipient server to resolve issue
Email throttling vs. deferral 

Receiving a deferral or a throttling message can be frustrating, especially if you have been doing everything correctly. However, regular error messages could mean deeper issues and harm your email marketing efforts.


The Impact of Email Throttling on Email Marketing

A throttling message is an alarm to stop and check for potential issues, including sender reputation and email engagement. A high number of throttling messages can lead to poor sender reputation, disrupting email deliverability.

A low deliverability rate, in turn, leads to low email open and click-through rates, and utlimately, a decline in email campaign performance

Here’s a closer look at the impact of throttling on email marketing campaigns:

  • Delayed or no delivery: Email throttling can pause your planned email campaigns, including time-sensitive promotions and event invites. In worse cases, your emails may be blocked from reaching the recipients.
  • Reduced reach: You may not be able to reach the entire audience, which can affect lead generation and email conversions
  • Decreased engagement: If you promised your subscribers a marketing newsletter every friday and it doesn’t reach them on time, they may lose interest eventually. Delays in email delivery can also cause them to unsubscribe altogether
  • Poor sender reputation: Consistent throttling messages can lead to reduced sender reputation and increase the chances of emails being filtered by spam folders.
  • Difficulty in testing and optimization: Delayed email delivery can make it challenging to A/B test email campaigns, making it difficult to gather timely feedback and improve campaign performance.

Email throttling is one of the most significant challenges in email marketing. However, once you understand how to navigate the process, you can turn it into an opportunity to improve email campaigns. 

To do that, it’s vital to understand how email speed and queueing might affect your ability to reach recipients on time. First, let’s cover the meaning of these terms to understand their comparison better.


What is the Email Journey?

An email or customer journey refers to the process of sending automated emails based on specific customer triggers. In simple terms, it’s an opportunity to continuously nurture your leads until they reach the final step in the buyer’s cycle.

Journeys can be triggered by various events, such as:

The goal is to provide relevant information and guide users through a specific path in the email marketing funnel. The email journey is a subset of customer journey and is used to automate processes from onboarding new customers to keeping them engaged.

To regularly keep customers engaged, your emails must reach them on time. This means your email speed must be at par to deliver thousands of emails at once.


What is Email Speed?

Email speed refers to the rate at which emails are delivered to recipient inboxes. While you may believe that inbox placement dictates email marketing effectiveness, how quickly your emails arrive is equally important.

Several factors can influence your email delivery speed, such as the following:

  • A good sender reputation shows ISPs that your emails are safe to deliver, increasing the email speed and deliverability percentage.
  • Having a clean email list with valid email addresses reduces bounces and improves email speed.
  • Emails with fewer image files and multimedia tend to deliver faster than big email sizes.
  • The infrastructure and capabilities of your ESP can also influence email speed.
  • Email throttling may cause a slowdown in email delivery and speed.

ISPs are good at distinguishing bulk messages from transactional emails. When they sense an email is going from a source to thousands of people at once, they may throttle for inspection. This may slow down the email delivery speed and affect deliverability rates. Sometimes, these emails are queued to be sent at a later time. Other times, you may receive a throttling error message.


What is Email Queueing?

When you send mass emails, not every email will be sent immediately. The emails are queued on an SMTP server that provides temporary storage before the emails can be sent. This queuing process helps manage email traffic efficiently.

Email queuing is important to prevent email servers from getting overloaded. Sometimes, email providers might send emails in batches and hold them in queues for smoother processes. 

You might encounter email queuing in the following scenarios:

  1. When you send large email campaigns: When you send mass emails, the emails can be queued until they are sent in batches.
  2. When there are network issues: If you have a weak internet connection, the emails might be queued until a stronger connection is found.
  3. When there are issues with the email server: Temporary issues with the email server may pause and queue your emails until resolved.

Understanding email queuing will help you manage expectations and implement the right strategies for timely delivery.


How Does Email Journey, Speed, and Queuing Affect Email Throttling?

The interplay between journey, speed, and queueing can impact how ISPs throttle emails. Each component can influence the other components and determine the success of your email marketing campaigns. 

In the table below, you’ll see how the three affect email throttling.

Aspect Factor How it Affects Email Throttling
Email journeySending frequencyBlasting emails to complete a journey can trigger throttling due to a sudden spike
Email journeyContent relevanceSending irrelevant or spammy content can lead to throttling and reduce sender reputations
Email journeyList hygieneBounced emails due to invalid email addresses can affect deliverability and contribute to throttling
Email SpeedSpeedSlower email speed due to larger email content or poor sender reputation might lead to emails lingering in queues and trigger throttling
Email queueing Sending burstsA large email campaign in a journey can overload the queue and initiate throttling if the ESP’s limits are exceeded
Email queueing Reputation bufferA good sender reputation can act as a buffer against throttling as long as the sending limits are moderate
Factors affecting email throttling

Although email speed and queueing can impact throttling, it ultimately comes down to your ESP’s infrastructure and sending limits. Most ESPs charge by the number of emails sent, which means there’s minimal focus on email deliverability.

However, a reliable ESP will prioritize inbox placement and sending speed to ensure flawless email marketing results. 


Super Fast Queuing and Sending With Campaign Refinery

Campaign Refinery is known for providing the highest inbox placement rate on the market. The platform has a built-in feature that lets clients send thousands of emails at high speeds without worrying about email delivery issues. Our sending engine batches content and delivers them all at once, regardless of your email list size. Sometimes, we have had to intentionally slow down the process to meet major mailboxes’ criteria. 

In short, Campaign Refinery has one of the fastest sending speed limits in the industry.

We achieve this by focusing on the following concepts:

  1. Emphasis on permission-based marketing:  Clients can migrate with opted-in email lists only. We handpick clients based on their email marketing history to determine compatibility. Once onboarded, the email deliverability team closely monitors the email volume and sending habits to ensure safer sending practices.
  2. Compliance with sending limits: With constant monitoring, we ensure senders don’t exceed the email send limit. 
  3. Content and performance: Sometimes, emails can get tanked due to poor email layout and improper email elements. Campaign Refinery provides built-in tools to craft compelling email messages and track results to refine future campaigns.
  4. Email list cleaning: Campaign Refinery offers an integrated email list cleaning feature that automatically removes harmful and invalid email addresses from your lists. This tool ensures cleaner lists and protects the sender’s reputation in the process.
  5. Email personalization: Generic emails are more likely to get throttled. Campaign Refinery clients can use dynamic fields, segment lists using tags, and hyper-personalize emails for higher email deliverability.

Alongside premium tools and features, we love to help marketers strengthen their email deliverability game. We have a unique guide to help boost your email open and click rates, ultimately getting you the results you expect.


Reach More Inboxes with the Inbox Formula

This comprehensive guide teaches you to master email-sending best practices and avoid spam traps. You’ll be able to reach more people, build sender reputation, and increase email campaign metrics

Download the Inbox Formula now!

If you’re wondering how this guide will help prevent throttling, here’s your answer:

  1. Improves open rates: The Inbox Formula helps you craft high-quality emails, which increases the likelihood of higher email open rates. Higher open rates translate to fewer throttling messages.
  2. Reduces spam complaints: The guide deep-dives into permission-based marketing frameworks and avoids spammy tactics. Fewer spam complaints can lead to a better sender reputation and fewer or no email throttling issues.
  3. Compliance with email sending best practices: The Inbox Formula highlights sending best practices, including proper email authentication and list hygiene. These practices contribute to smoother delivery and mitigate the risks of throttle. 

While the Inbox Formula offers valuable insights on improving email deliverability and sender reputation, it’s crucial to use a reliable ESP to help you send bulk emails safely. 

Experience supercharged email deliverability and fast sending speed with Campaign Refinery — Sign up and become a client today!

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