What is Email Throttling?

Email throttling controls the rate at which emails are sent to prevent overloading servers, protect the sender’s reputation, and avoid spam filters. Essential for marketers and businesses, it ensures effective delivery and smooth management of email campaigns.


Ever get a bounce message about a full mailbox? That’s email throttling in action. It’s a temporary roadblock, usually fixed within 72 hours.


How do you know if your emails are being throttled?

The refusal to deliver your email is usually accompanied by error messages, for example:


  • User is receiving mail at too great a rate right now


  • Mail undelivered


  • User’s mailbox is over quota


  • You have reached your daily sending quota


While email throttling, marked by a 400 Bad Request error, can be frustrating for marketers, sometimes, these delays even offer unexpected benefits. They can help to your delivery rates by keeping spam filters happy.



Causes of Email Throttling

ISPs, the internet service providers that deliver your emails, have limits on how many emails they’ll accept from a sender in a specific timeframe. If you exceed these limits, your emails bounce back.


These limits are based on your sender reputation, which considers factors like your IP address, any abuse complaints against you, the bounce rate of your emails (how many are undeliverable), how engaged your subscribers are (do they open your emails?), and any spam reports. For instance, a new IP address will likely have lower daily sending limits until it builds a good reputation with ISPs.


Here’s why an ISP might reject your emails:


  • Spam-Like Sending: Too many emails too quickly can flag your messages as spam.


  • Dirty List: Sending to inactive or non-existent addresses can trigger rejections.


  • Spam Reports: Recipients marking your emails as spam may cause temporary rejections.


  • New IP Limits: A new IP might face stricter limits until it earns a good reputation.


  • Full Inbox: The recipient’s mailbox is full. Blocked Ports: ISP servers may lack open ports for incoming emails.




How Does Email Throttling Work?

Email throttling paces your messages to avoid inbox overload. Here’s how it works:


  • Rate Limiting: Think of it like a traffic light for emails. You can only send a certain number per minute, hour, or day.


  • Batching: Break up your list and send emails in smaller groups, with a pause in between.


  • Domain Limits: Going heavy on a single domain (like spike.chat)? Throttling paces emails to avoid overwhelming their server.



Why Email Throttling is Important

Email throttling is crucial for businesses because it directly impacts three key aspects of email marketing: spam filtering, deliverability and sender reputation:

  1. Reduced Spam Filtering:

    Large email volumes sent quickly can trigger spam filters used by services like Gmail, Yahoo, and Spike Teamspace. Throttling helps avoid this by distributing the email load over time, ensuring your messages reach inboxes instead of spam folders.

  2. Improved Sender Reputation:

    Email servers maintain a reputation score for each sender based on factors like bounce rates, spam complaints, and sending volume. A high sender reputation ensures messages are delivered to inboxes. Throttling helps maintain a good reputation by controlling the sending volume and minimizing bounces and complaints.

  3. Protecting Deliverability:

    Sending too many emails at once can overwhelm both the sender’s and recipient’s email servers. This can lead to delayed delivery or even server crashes. Throttling helps manage the email flow, ensuring smooth operation for both sending and receiving servers.


Downsides of Email Throttling

While email throttling offers significant advantages, it’s not without its drawbacks:

  • Implementation Complexity:

    Effectively configuring throttling parameters necessitates a clear understanding of both email infrastructure and recipient behavior patterns. This can be challenging for those unfamiliar with email delivery processes.

  • Potential Delivery Delays:

    Throttling can introduce delays in the overall email delivery process, particularly for large mailing lists. This might not be ideal for time-sensitive communications, where prompt delivery is crucial.

  • Balancing Sending Volume:

    Finding the optimal sending volume can be a delicate balancing act. Sending too few emails may limit campaign reach, while sending too many can trigger spam filters or overwhelm recipient servers. This requires ongoing monitoring and adjustments to achieve the desired outcome.


How to Implement Email Throttling

How do you actually put throttling into action? Here are your options, from easiest to most technical:



Automatic Email Service Provider Configuration:

Most email service providers (like Mailchimp, SendGrid, or Constant Contact) have throttling built right in. You can set limits and schedules to keep your email flow smooth.



SMTP Settings:

If you run your own email server, you can adjust the settings on your SMTP (Simple Mail Transfer Protocol). This lets you control how many connections you make, recipients you send to per email, and the overall sending speed.



Custom Scripts and APIs:

Advanced users (or those with very specific needs), you can create custom scripts or use email-sending APIs to throttle emails. This option offers the most flexibility, but requires some coding know-how.



How to Prevent Email Throttling

To maintain a strong sender score and avoid email throttling, consider these proactive strategies:



Safeguard Your Sender Reputation

Maintaining a strong sender reputation is crucial for ensuring your emails reach intended inboxes. Building trust takes time, and email reputation is no exception. When sending from a new IP or domain, don’t bombard recipients with emails. Warm up the IP by gradually increasing sending volume over a period (ideally 30 days). This establishes a positive sending history and allows for future increases as your campaigns require.



Segment Your Lists:

Similar to warming up your IP, consider segmenting your email traffic. Separate marketing and transactional emails, allowing you to build and maintain independent reputations for each list. This targeted approach helps avoid overwhelming recipients and improves overall deliverability.



Regularly clean your email databases.

Remove inactive, invalid, and unsubscribed addresses. A clean and active list reduces bounce rates, a key factor in maintaining a good sender reputation and preventing throttling.



Use Email Automation Tools:

Simplify email throttling management with automation tools. These tools often set daily sending limits or introduce delays between messages, preventing bursts that trigger throttling.


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