What is email throttling?

Email throttling is a mechanism used by mailbox providers to limit the number of emails an IP address or domain can send within a given timeframe. It's often employed to prevent spam, manage server load, and protect recipient inboxes from overwhelming volumes, especially from new or untrusted senders.

Why do new IPs and domains experience more throttling?

New IPs and domains lack an established sending history and reputation. Mailbox providers are cautious of new senders to prevent spam, so they impose lower limits initially. Gradually increasing your sending volume (warming up) helps build trust and signals that you are a legitimate sender.

How do I warm up my IP and domain to avoid throttling?

Start by sending small volumes to your most engaged subscribers. Gradually increase volume over days or weeks, watching your deliverability metrics. If you see throttling, reduce volume and slow down the warm-up process. Consistency is key, avoiding large, sudden spikes in volume.

What are the key indicators of email throttling?

Monitor your SMTP logs for temporary errors (4xx codes), which indicate throttling. Pay attention to bounce rates and complaint rates, as high numbers will lead to increased throttling and reputation damage. Using Postmaster Tools can provide valuable insights .

Are there specific message limits per minute or hour for major recipient domains?

While there are no publicly available universal limits, some providers, like Orange.fr and Wanadoo.fr, are known for strict connection limits. It's more effective to respond dynamically to error codes and gradually increase volume rather than attempting to find fixed numbers. Most providers prioritize sender reputation.

Do bounce and complaint rates affect throttling?

Yes, high bounce rates (due to invalid addresses) and high complaint rates (users marking your email as spam) significantly damage your sender reputation. This often leads to more aggressive throttling or even blacklisting (also known as a blocklist), making it harder to reach the inbox. Regularly cleaning your list helps mitigate these issues.

What are the best practices for handling throttling from new IPs and recipient domains? - Sender reputation - Email deliverability - Knowledge base

Dealing with email throttling, especially when using new IP addresses or sending to new recipient domains, is a common challenge for anyone managing email delivery. It’s a clear sign that an inbox provider is limiting the volume or rate of emails you can send, often to protect their users from potential spam or to manage server load. This usually manifests as temporary errors, like a "451 4.7.1 Received too many messages from a new or untrusted IP" response, indicating that your sending practices are being flagged as suspicious or overly aggressive.

The core issue here often boils down to trust. When you introduce a new IP or begin sending to a domain you haven’t interacted with extensively, you lack an established sender reputation. Inbox providers like

Google and

Microsoft (and many others) employ sophisticated algorithms to evaluate senders, and new IPs or domains simply haven't had enough time to build that trust. This article will outline some best practices to navigate these waters.

Successful email delivery relies heavily on a careful approach, particularly when you're working with fresh infrastructure. It’s not just about getting past a temporary block, but about strategically building a positive sending history that will lead to consistent inbox placement.

Building sender reputation

The foundation of avoiding throttling is a solid IP and domain reputation. This is built over time through consistent, high-quality sending. For new IPs or domains, this process is known as warming up. The goal is to gradually increase your sending volume to a diverse set of engaged recipients, demonstrating to inbox providers that you are a legitimate sender.

A key aspect of this is starting small. Instead of immediately sending large volumes, begin with a low volume of emails to your most active and engaged subscribers. As your emails are consistently delivered and opened, and as complaint rates remain low, you can slowly escalate your sending volume. You can find more detail on this process in our guide on dedicated IP warm-up.

Consistency is also crucial. Irregular sending patterns, especially large spikes after periods of inactivity, can trigger throttling. Aim for a steady increase in volume rather than sporadic bursts. This predictable behavior helps maintain a positive perception with mailbox providers.

Aggressive sending

Strategy: Sending large volumes immediately to new IPs or domains.
Reputation Impact: High risk of being flagged as spam, leading to immediate throttling or blacklisting. Your IP (or blocklist) reputation will suffer.
Outcome: Poor deliverability, emails landing in spam folders, and potential long-term damage to sender trust.

Understanding recipient-specific limits

One of the most complex aspects of email deliverability is that each recipient domain, and indeed each Mail Exchange (MX) server, might have its own unique throttling policies. There isn't a universal guide detailing exact message limits per minute or hour for every domain. These limits are often dynamic, adjusting based on your sender reputation, current network load, and other proprietary factors.

For instance, while

Yahoo might suggest a maximum of 20 messages per SMTP connection, this isn't a hard rule for all their systems, nor does it define per-minute or per-hour limits. French ISPs like Orange.fr and Wanadoo.fr are known to have strict throttling, often limiting the total concurrent connections across both domains to as few as three. Our guide on warming email sending IPs by individual ISP provides more context.

This variability means that manual guesswork, where you try different sending rates until deliverability improves, isn't the most efficient or reliable strategy. It often leads to further throttling or even being put on a blacklist (or blocklist), delaying your progress. You need to respect the dynamic nature of these limits and adjust accordingly.

Understanding how message rate limits and throttling are typically managed by major providers like

Outlook can help inform your strategy.

Implementing smart throttling mechanisms

To effectively handle throttling, you need to implement intelligent throttling mechanisms on your sending infrastructure, whether it's an in-house Mail Transfer Agent (MTA) or a service. The goal is to dynamically adjust your sending rate based on feedback from recipient servers.

Instead of focusing strictly on messages per minute or hour, it's often more effective to manage the number of concurrent connections and messages per connection. Many inbox providers prefer a higher number of messages per connection (e.g., 500 messages) over frequent connection and disconnection cycles. This minimizes overhead for both sides and indicates a more stable sending pattern. You can explore this further in our discussion on daily or hourly throttling.

Automated back-off mechanisms are critical. When you receive a transient error code (like a 4xx series SMTP response), your system should automatically pause sending to that specific domain or IP and retry later, gradually increasing the delay between retries. This signals to the receiving server that you are a respectful sender. If you're building an in-house MTA, implementing these dynamic rules is complex but essential for long-term success.

Monitoring and adaptation

Even with the best warming practices and throttling logic, continuous monitoring is non-negotiable. Watch your delivery logs closely for deferred messages, temporary failures, and error codes. Tools like

Amazon SESprovide guidance for warming and migration, which emphasizes a gradual approach.

Pay particular attention to your complaint rates and bounce rates. High numbers here will swiftly lead to throttling, blacklisting (or blocklisting), and severe reputation damage. Cleaning your email lists regularly to remove inactive or invalid addresses is a proactive measure against these issues. Consider setting up a blocklist monitoring service as well.

The key is adaptability. If you notice persistent throttling from a specific domain, it’s a signal to reduce your sending volume to that domain and re-evaluate your strategy. Sometimes, backing off to a previously successful sending level for a few days can help reset the perception of your IP.

Summary of best practices

Managing email deliverability, especially with new IPs and recipient domains, requires a strategic, patient, and adaptive approach. Focusing on building a strong sender reputation through proper IP and domain warming, understanding the nuances of different ISP throttling policies, implementing smart sending mechanisms, and continuously monitoring your performance will put you in the best position for consistent inbox delivery. Remember, it's a marathon, not a sprint.

Views from the trenches

Best practices

Always warm up new IP addresses and sending domains gradually, starting with small volumes of highly engaged recipients.

Prioritize recipient engagement and low complaint rates to build and maintain a positive sender reputation over time.

Implement dynamic throttling logic in your MTA to automatically adjust sending rates based on recipient feedback and error codes.

Focus on sending more messages per SMTP connection rather than frequently opening and closing connections to a recipient's MX.

Regularly clean your email lists to remove invalid or unengaged addresses, reducing bounces and spam trap hits.

Common pitfalls

Sending large, untargeted volumes of email immediately from a new IP or domain, triggering instant throttling.

Ignoring temporary delivery errors (4xx SMTP responses) and continuing to send at the same rate.

Assuming a fixed, universal throttling limit across all recipient domains, leading to over-sending or under-sending.

Neglecting sender reputation metrics like bounce and complaint rates, which are key indicators for throttling.

Attempting to manually guess optimal sending rates without relying on automated feedback loops from recipient servers.

Expert tips

Consider segmenting your email campaigns to separate highly engaged audiences from less active ones, especially during warm-up.

Utilize dedicated IPs for different types of email traffic, like transactional versus marketing emails, to protect reputation.

Maintain strong authentication (SPF, DKIM, DMARC) for your sending domains to establish credibility with recipient servers.

Leverage Postmaster Tools from major ISPs like Google and Microsoft for insights into your sending performance and reputation.

If throttling occurs, immediately reduce sending volume to the affected domain and slowly re-introduce traffic after a few days.

Expert view

Expert from Email Geeks says that the number of messages allowed per minute and per hour are not fixed by Yahoo. They are dynamic.

2020-12-15 - Email Geeks

Expert view

Expert from Email Geeks recommends backing off to a previously successful sending level and maintaining that for 2-3 days if you are experiencing throttling.

2020-12-15 - Email Geeks

Navigating throttling for deliverability

Navigating email throttling from new IPs and recipient domains is fundamentally about establishing and maintaining trust with mailbox providers. It requires a well-planned IP warming strategy, an understanding of how different recipient domains behave, and robust technical implementations that can dynamically respond to throttling signals. Always remember that your sending speed is ultimately dictated by what recipients are willing to accept, not by how fast your system can send.

By adhering to best practices like gradual volume increases, consistent sending, proper authentication, and continuous monitoring, you can effectively manage throttling and ensure your emails reach their intended inboxes, building a strong and reliable sender reputation over time. This approach helps avoid the pitfalls of blacklisting (or blocklisting) and ensures long-term deliverability success.