Email Throttling Explained: Why It Happens and How to Prevent It

Estimated reading time: 6 minutes

Email deliverability challenges are not always caused by spam filters or technical errors. In many cases, messages fail to arrive on time simply because they are being slowed down by the receiving server. This process is known as email throttling, and it is one of the most common reasons email campaigns experience unexpected delivery delays.

What Does Throttling Mean in Email Delivery?

In the context of email, throttling refers to the temporary limitation of sending speed imposed by a recipient’s mail server.

Rather than rejecting a message entirely, the receiving server delays acceptance by restricting how many emails it will process from a specific IP address or domain within a given time period.

In practical terms, the server is signaling: delivery is possible, but the sending rate must be reduced.

Throttling is therefore considered a soft restriction, not a permanent block.

It is designed to:

• Prevent inbox overload
• Reduce unsolicited bulk email
• Encourage responsible sending behavior
• Protect recipients from spam-like patterns

Throttling is especially common in high-volume email environments, including marketing campaigns, newsletters, and transactional messaging systems.

Why Do Recipient Servers Throttle Emails?

ISPs throttle emails for several reasons, most of which relate to trust, volume, and reputation.

1. Unusually High Sending Volume.

A sudden increase in email volume is often interpreted as suspicious behavior.

For example, sending tens of thousands of messages within a short period (especially after weeks of inactivity) may trigger rate limits.

2. Weak Sender Reputation.

Mailbox providers evaluate sender reputation based on factors such as:

• Bounce rates
• Spam complaints
• Engagement levels
• Historical sending behavior

If reputation declines, throttling becomes more likely.

3. New or Unfamiliar IP Addresses.

New sending IPs have no established track record. As a result, recipient servers may apply stricter limits until consistent, trustworthy sending patterns are demonstrated.

4. Authentication or Policy Issues.

Missing or misconfigured authentication protocols increase the likelihood of throttling, including:

• SPF
• DKIM
• DMARC

These standards help recipient servers verify that your messages are legitimate.

5. Server Capacity and Traffic Management.

Throttling may also occur due to temporary server-side conditions unrelated to your sending practices, such as high inbound traffic or system load.

6. The “Black Friday” Effect: Throttling During Peak Seasons.

It is important to note that throttling isn’t always triggered by your behavior alone. During high-traffic periods — such as Black Friday, Cyber Monday, or major holidays — mailbox providers like Gmail and Outlook become significantly more aggressive with their filtering.

Because the global volume of spam and marketing mail skyrockets during these windows, ISPs lower their “trust thresholds.” Even if your reputation is solid, a sudden spike in your volume during a holiday sale can trigger a throttle simply because the ISP’s infrastructure is under heavy load. To survive these peaks, it is vital to start increasing your volume weeks in advance rather than waiting for the day of the event.

“IPs Were Throttled by Recipient Server”: What It Means

The response “IPs were throttled by the recipient server” indicates that the receiving mail system has placed a temporary limit on your sending IP.

This typically means:

• Some messages were accepted successfully
• Additional messages were deferred
• The server expects retries at a slower pace

While not a hard bounce, this should be treated as an important deliverability signal.

Technical Signals: Understanding SMTP Error Codes

When a server throttles your connection, it doesn’t just stop sending data; it communicates the delay through specific SMTP response codes. 

Recognizing these codes in your delivery logs is the first step toward fixing the issue.

421 Code: This is the most common throttling signal. It often appears as “421 4.7.0 Temporary System Problem or 421 Request refused; try again later.” It tells your server that the recipient is currently unwilling to accept more mail from your IP.

451 Code: This indicates a “Local Error” or “Requested action aborted.” Many ISPs, such as Outlook or Yahoo, use this to signal that your sending limit has been reached for the current hour or day.

Greylisting: Some servers use a specific type of throttling called greylisting, which issues a temporary 4xx error to any unknown sender. A legitimate mail server will automatically retry after a few minutes, at which point the message is accepted.

Why Throttling Should Be Taken Seriously

One important aspect of email throttling is that it often occurs gradually and may not be immediately visible without careful monitoring. Many senders assume that as long as emails are not bouncing, delivery is functioning normally. However, throttling can quietly delay messages for hours, reducing the effectiveness of time-sensitive campaigns such as promotions, product launches, or transactional notifications. 

In these situations, even a temporary slowdown can lead to lower open rates, missed conversions, and inconsistent customer experience. Throttling can also create uneven delivery patterns, where some recipients receive messages promptly while others receive them much later, which may affect engagement metrics and further influence sender reputation. 

How to Reduce or Prevent Email Throttling

Although throttling is common, it can often be minimized through proper sending practices.

Adjust Sending Rates

Avoid sending large volumes in short bursts. Gradual distribution over time reduces the likelihood of triggering limits.

Warm Up New IP Addresses

Dedicated IPs should be introduced slowly, with sending volume increasing incrementally over several days or weeks.

Maintain Clean Subscriber Lists

Poor list hygiene increases bounce rates and complaints, both of which contribute to throttling.

Strengthen Domain Authentication

Ensure SPF, DKIM, and DMARC are correctly configured to establish credibility with ISPs.

Test Deliverability Regularly

Deliverability testing is especially important before high-volume sends. Tools like GlockApps provide visibility into spam placement, reputation trends, and delivery barriers across major providers.

The GlockApps delivery reports show a lot of details about the email performance including the delivery duration. This allows you to find out if your domain or IP experience email throttling issues. Email delivery used to take minutes; if it takes hours, the messages are likely to be throttled by the receiving provider.

Conclusion

Email throttling is a common deliverability mechanism used by mailbox providers to regulate email traffic and reduce unwanted sending behavior. While it does not always indicate a serious issue, throttling should be viewed as a signal that sending volume, reputation, or authentication practices may require adjustment.

By implementing responsible sending strategies (including gradual volume increases, proper authentication, list hygiene, and ongoing monitoring)  you can reduce throttling and ensure more reliable inbox placement over time. 

GlockApps can help identify inbox placement issues and detect early warning signs before throttling becomes persistent.

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