What factors influence ESP email sending speed benchmarks?
Matthew Whittaker
Co-founder & CTO, Suped
Published 19 Jun 2025
Updated 19 Aug 2025
6 min read
When considering email sending speed, it is common to look for benchmarks. How long should it take your Email Service Provider (ESP) to send one million emails, for example? This question, while seemingly straightforward, opens up a complex discussion about factors far beyond an ESP's raw technical capacity.
The reality is that ESP email sending speed benchmarks are not universal and depend heavily on numerous variables. While an ESP’s infrastructure plays a role, the dominant factors are often tied to your sender reputation, the specific internet service providers (ISPs) you are sending to, and the quality of your email lists and content. Understanding these influences is key to setting realistic expectations and optimizing your email program for efficient delivery.
The underlying power of mail transfer agents (MTAs)
Mail Transfer Agents (MTAs), the software responsible for sending emails, are inherently capable of incredibly high throughput. Modern MTAs like PowerMTA, Momentum, or MailerQ can theoretically push millions of emails per hour. However, this raw capability rarely translates directly to actual sending speeds observed by users.
ESPs typically throttle or pause email delivery to align with best practices and the varying acceptance policies of different ISPs. This intentional limitation prevents a sudden flood of emails from overwhelming receiving servers or triggering spam filters. An ESP's internal queueing mechanisms might process messages quickly, but the true delivery speed is dictated by external factors and how mail is accepted by the destination.
Even with powerful underlying infrastructure, an ESP's ability to send mail quickly is ultimately constrained by how Mailbox Providers (MBPs) like Mailchimp handle large volumes. The goal is not just to send fast, but to send reliably to the inbox, which often means adhering to lower, sustainable rates.
Sender reputation and ISP throttling
The single most significant factor influencing your actual email sending speed is your sender reputation. ISPs evaluate your sender reputation based on engagement rates, spam complaints, bounce rates, and whether your IP address or domain is on any email blocklists (or blacklists). A strong reputation allows for higher sending volumes and faster delivery. Conversely, a poor reputation will lead to severe throttling, delays, and even blocking.
ISPs actively manage incoming email traffic. If they detect unusual sending patterns, high complaint rates, or bounces from your IP or domain, they will initiate throttling. This means they will temporarily slow down or stop accepting emails from you. The goal for ISPs is to protect their users from unwanted mail, and a sudden surge or consistent flow of low-quality mail is a red flag. Sender reputation is critical in this dynamic.
It is also worth noting that different ISPs have different acceptance policies and capacities. Sending to Gmail or Outlook (or Yahoo Mail) in the US might yield different speeds compared to sending to smaller, regional ISPs or those in countries with stricter email policies, like China or Poland.
Good sender reputation
High engagement: Recipients consistently open, click, and reply to your emails.
Low complaints: Minimal spam reports from recipients.
Clean list: Few invalid addresses, bounces, or spam traps.
Proper authentication: Correctly configured SPF, DKIM, and DMARC.
Impact on sending speed
Faster delivery: ISPs accept higher volumes of mail, leading to quicker campaign completion.
Less throttling: Fewer rate limits or temporary rejections imposed by receiving servers.
Improved inbox placement: Emails are more likely to reach the inbox, not spam folders.
Poor sender reputation
Low engagement: Emails are ignored or deleted without opening.
High complaints: Frequent reports of emails being spam.
Dirty list: Includes spam traps, old or invalid addresses.
Authentication issues: Incorrect or missing SPF, DKIM, DMARC records.
Frequent throttling: Campaigns take significantly longer to send, if they complete at all.
Spam folder placement: Many emails are diverted from the inbox, impacting reach.
Ultimately, if you experience excessively slow sending speeds, such as 100,000 emails taking 12 hours or more, it is almost always indicative of a reputation issue rather than a limitation of your ESP's core infrastructure.
Infrastructure, message complexity, and list quality
While sender reputation is paramount, an ESP's technical infrastructure also plays a role. The underlying servers, network configuration, and even the versions of Mail Transfer Agents (MTAs) used can subtly impact sending speed. Older MTA versions, for instance, might not handle high-volume TLS (Transport Layer Security) encryption efficiently, leading to transient errors and network resets that slow down delivery.
The complexity and size of your email messages can also influence speed. Emails with numerous large images, complex HTML structures, or extensive dynamic content require more processing power and bandwidth, both from your ESP and the receiving ISP. Features like DKIM signing, while essential for authentication, also consume CPU resources, especially with larger keys.
The quality of your email list is a foundational element that affects not only deliverability but also sending speed. Sending to outdated, unengaged, or bought lists leads to high bounce rates, spam complaints, and hits to your sender reputation. ISPs will quickly throttle or block mail from senders with consistently poor list hygiene, directly impacting how fast your campaigns can be processed and delivered.
Best practices for optimizing sending speed
Maintain a clean list: Regularly remove inactive subscribers and hard bounces.
Segment your audience: Send targeted emails to highly engaged segments first.
Optimize content: Compress images and avoid overly complex HTML.
The quest for a universal ESP email sending speed benchmark is often futile because the "real" number is so dependent on various factors specific to each sender and message. Publicly available, normalized benchmarks are rare precisely because of this variability. While ESPs might quote average messages per hour during sales cycles, these figures are often theoretical maximums or averages based on optimized conditions, not guarantees for every client.
It is crucial to differentiate between mail that is "queued for delivery" by an ESP and mail that is "actually delivered" to recipient inboxes. An ESP can queue millions of emails very quickly, but the actual delivery speed is contingent upon ISPs accepting that mail. If ISPs are throttling your sends due to reputation issues, the queued emails will sit in a deferral queue, waiting for acceptance, leading to significant delays.
Furthermore, the destination of your emails plays a critical role. Sending to high-volume ISPs like Google might result in faster delivery rates due to their infrastructure and policies compared to sending to smaller, less robust, or geo-restricted email providers.
Factor
Impact on sending speed
Sender reputation
The primary determinant. High reputation = faster delivery. Low reputation = throttling and delays.
ISP acceptance policies
Different ISPs have varying rate limits and acceptance criteria based on sender behavior.
Email content complexity
Large images, heavy HTML, and dynamic content can slow down processing.
List quality
Sending to invalid addresses or spam traps leads to bounces and complaints, which reduces speed.
ESP infrastructure
While powerful, some older MTA versions or misconfigurations can cause minor delays.
Views from the trenches
Best practices
Focus on maintaining an excellent sender reputation, as this is the primary driver of sending speed and deliverability.
Regularly clean your email lists to remove inactive or invalid subscribers, minimizing bounces and complaints.
Segment your audience and prioritize sending to highly engaged subscribers to build a positive sending history.
Common pitfalls
Focusing solely on an ESP's advertised maximum sending capacity without considering real-world deliverability factors.
Ignoring list hygiene, leading to high bounce rates and spam complaints that severely slow down sending.
Sending large volumes of emails to unengaged segments, which triggers ISP throttling and negatively impacts reputation.
Expert tips
Remember that email is a 'store and forward' system, meaning messages are queued and sent when accepted, not instantly delivered.
Focus on the quality of your sending practices and data, as this matters far more than the raw speed of your ESP.
The maximum sending speed of a Mail Transfer Agent (MTA) is often limited by receiving ISPs' acceptance policies, not the MTA's capacity.
Marketer view
Marketer from Email Geeks says that while ESPs can queue mail very quickly, the actual delivery speed is often much slower due to ISP acceptance.
2019-09-03 - Email Geeks
Expert view
Expert from Email Geeks says that the send rates vary significantly by IP address and sender, not generally by the ESP itself, as most MTAs can send millions per hour if reputation is not an issue.
2019-09-03 - Email Geeks
Prioritizing deliverability over raw speed
While it is tempting to seek concrete ESP email sending speed benchmarks, the true measure of efficient email delivery lies in optimizing your overall email program. Raw sending capacity is less important than your ability to maintain a strong sender reputation, adhere to ISP guidelines, and manage a high-quality email list.
Focusing on these foundational elements will naturally lead to faster and more reliable email delivery, ensuring your messages reach the inbox promptly and efficiently, regardless of theoretical speed limits.
Gmail or 
Outlook (or 
Yahoo Mail) in the US might yield different speeds compared to sending to smaller, regional ISPs or those in countries with stricter email policies, like China or Poland.
Google might result in faster delivery rates due to their infrastructure and policies compared to sending to smaller, less robust, or geo-restricted email providers.
