Email Sending Speed Best Practices for Deliverability
Matthew Whittaker
Co-founder & CTO, Suped
Published 14 Aug 2025
Updated 19 Aug 2025
7 min read
Email sending speed is a crucial, yet often overlooked, factor in achieving optimal email deliverability. It's not just about how many emails you can send, but how you send them, especially when interacting with major mailbox providers. Sending too quickly can flag your activity as suspicious, leading to throttling or outright blocking, while sending too slowly might delay your communications unnecessarily.
The ideal sending speed is a delicate balance, influenced by various factors including your sender reputation and the specific policies of the receiving servers. Understanding and optimizing this balance is key to ensuring your messages consistently reach the inbox, avoiding spam folders and common blocklists (or blacklists).
Mailbox provider rate limits
Mailbox providers, such as Gmail and Outlook, implement rate limits to protect their users from spam and abuse. These limits dictate how many connections an IP address can open, how many recipients it can send to per connection, and how many emails can be accepted within a specific timeframe, often per minute or hour. These aren't static rules, they vary dynamically. What's acceptable for one mailbox provider might be seen as aggressive by another. You can learn more about acceptable email sending speeds and throttling.
When you exceed these invisible thresholds, mailbox providers may respond by temporarily rejecting your emails with specific error codes, a process known as throttling. This is a clear signal that your sending rate is too high for your current sender reputation or for that specific domain at that moment. Continuous disregard for these signals can lead to more severe consequences, such as permanent blocks or placement on a blocklist (or blacklist).
The key numbers to watch are the number of open connections to each mail exchanger (MX) and the number of recipients per minute over each connection. These metrics are often found in your email logs. Understanding and reacting to these fluctuating limits is vital. For more on this, consider our guide on how to handle email sending rate and connection limits.
Building and maintaining sender reputation
Your sender reputation is arguably the most critical factor influencing your allowed sending speed. A strong, positive sender reputation tells mailbox providers that you are a legitimate sender sending desired mail. This trust allows you to send emails at higher volumes and speeds without encountering throttling or filtering issues. Conversely, a poor reputation will drastically reduce your allowed sending speed and increase the likelihood of your emails landing in the spam folder or being rejected outright. Factors such as spam complaints, bounce rates, and engagement metrics (opens, clicks) all contribute to your reputation. To understand this better, read about how email sending practices impact domain reputation.
Building and maintaining a good reputation is an ongoing effort. It involves consistently sending relevant and engaging content to an interested audience, managing your email list effectively, and ensuring proper email authentication. According to Twilio SendGrid, a sender's reputation is a critical set of metrics influencing deliverability. Your sender score directly correlates with your ability to send at higher volumes.
Modern email filters increasingly factor reputation into their rate limits. A sender with a strong reputation might be allowed to send millions of emails daily, while one with a mediocre reputation could be capped at a much lower volume. This highlights the importance of fostering positive sender behavior rather than solely focusing on technical sending speeds. Learn more about the best email sending speed to avoid spam folders.
Key factors for good sender reputation
Low bounce rates: Regularly clean your email lists to remove invalid or inactive addresses.
High engagement: Send relevant content that encourages opens and clicks.
Low complaint rates: Make it easy for subscribers to opt-out, and avoid sending unsolicited emails.
Consistent sending volume: Maintain a steady sending cadence to build a predictable pattern with mailbox providers.
Strategic sending and IP warming
When you're starting with a new IP address or domain, or if you've been inactive for a while, IP warming is essential. This involves gradually increasing your email sending volume over days or weeks to build a positive sending history and reputation with mailbox providers. Starting with small batches of highly engaged recipients and slowly expanding helps establish trust and avoids triggering spam filters. MailerLite recommends gradually increasing the number of emails you send over several weeks.
Even with an established reputation, maintaining a consistent sending volume and schedule is beneficial. Erratic sending patterns can look suspicious to mailbox providers. For instance, sending a massive blast after a period of inactivity can negatively impact your reputation and deliverability. According to Mailchimp, it's best to start with small batches and avoid sending bulk emails all at once.
When sending from a new IP address or domain, start with very small volumes to your most engaged subscribers. Gradually increase the volume over several weeks. This process, known as IP warming, builds trust with mailbox providers and establishes your sending reputation from the ground up.
Start small: Begin with hundreds, then thousands, of emails daily.
Engaged audience: Focus on subscribers likely to open and click.
Consistency: Maintain daily sending throughout the warming period.
Established IP or domain
For warmed IPs and established domains, the focus shifts to maintaining consistency and responding to mailbox provider feedback. While you can send higher volumes, avoiding sudden spikes or drops is crucial. Your reputation allows for more flexibility, but radical changes can still trigger filters.
Steady volume: Aim for predictable sending patterns, even with high volumes.
Monitor feedback: Regularly check logs and postmaster tools for throttling signals.
Segment campaigns: Send targeted emails to maintain engagement and prevent over-sending to less active users.
Monitoring and adjusting sending speed
Effective monitoring is crucial for optimizing your email sending speed. This involves regularly reviewing your email logs for soft bounces or temporary rejections related to rate limits. Many mailbox providers provide specific error codes that indicate throttling. By understanding these signals, you can dynamically adjust your sending rates to align with what each provider is willing to accept.
Postmaster tools from major mailbox providers offer valuable insights into your sending reputation and potential issues. For example, Google Postmaster Tools provide data on IP and domain reputation, spam rates, and delivery errors. Using these tools allows you to proactively identify and address problems before they significantly impact your deliverability. For more on this, check out our guide on how to improve domain reputation using Google Postmaster Tools. Additionally, Klaviyo suggests using a sending schedule based on email engagement, a crucial factor in maintaining a positive relationship with subscribers.
The insights gained from monitoring should inform your adjustments. If you see persistent throttling for a specific provider, consider reducing your connection or recipient limits for that domain. Conversely, if you're consistently achieving high delivery rates without issues, you might gradually increase your speed. This iterative process of monitoring and adjusting is key to long-term deliverability success, as discussed in detail on HostAdvice's deliverability overview. Understanding what factors influence ESP email sending speed benchmarks will also help you fine-tune your approach.
Views from the trenches
Conclusion
Managing email sending speed for deliverability is a dynamic and multifaceted challenge. It's not about finding a single 'magic number,' but rather understanding the intricate relationship between your sending practices, sender reputation, and the varying requirements of mailbox providers. Prioritizing a positive sender reputation through consistent engagement and list hygiene will inherently allow for greater sending flexibility.
Successful email deliverability hinges on a polite and adaptive sending strategy. By monitoring your logs, leveraging postmaster tools, and adjusting your sending speed based on feedback, you can build and maintain the trust necessary to consistently reach your subscribers' inboxes.
Gmail and 
Outlook, implement rate limits to protect their users from spam and abuse. These limits dictate how many connections an IP address can open, how many recipients it can send to per connection, and how many emails can be accepted within a specific timeframe, often per minute or hour. These aren't static rules, they vary dynamically. What's acceptable for one mailbox provider might be seen as aggressive by another. You can learn more about acceptable email sending speeds and throttling.
Google Postmaster Tools provide data on IP and domain reputation, spam rates, and delivery errors. Using these tools allows you to proactively identify and address problems before they significantly impact your deliverability. For more on this, check out our guide on how to improve domain reputation using Google Postmaster Tools. Additionally, Klaviyo suggests using a sending schedule based on email engagement, a crucial factor in maintaining a positive relationship with subscribers.
