Summary
The speed at which an email service provider (ESP) can send a large volume of emails is a common question, yet the answer is far from simple. While the underlying Mail Transfer Agent (MTA) software is capable of very high throughput, the actual sending speed is heavily influenced by a myriad of factors, primarily revolving around sender reputation, recipient Mailbox Provider (MBP) policies, and the ESP's infrastructure and practices. Benchmarking email sending speeds is complex because hypothetical maximums rarely reflect real-world delivery times, which are often intentionally throttled for optimal deliverability.
Key findings
- Intentional throttling: Most ESPs deliberately limit their sending speed to adhere to mailbox provider best practices, preventing mail from being blocked or sent to spam. This is a critical factor for maintaining good sender reputation.
- Reputation dependence: The primary determinant of actual delivery speed to inboxes is the sender's reputation (IP and domain). A strong reputation allows for faster delivery, while a poor one leads to throttling or blocking.
- ISP-specific limits: Mailbox providers like Google, Microsoft, and Yahoo have varying acceptance rates and connection limits that directly impact how quickly emails can be accepted. These limits can vary significantly by region or even individual sender reputation.
- MTA capabilities: While MTA software (e.g., PowerMTA, Momentum) can theoretically send millions of emails per hour, ESPs configure them to prioritize deliverability over raw speed, often influenced by the sender's practices and message content.
Key considerations
- Focus on deliverability, not just speed: Pure sending speed benchmarks can be misleading; the ultimate goal is inbox placement, which often requires a slower, more controlled sending pace.
- Sender reputation is paramount: Improving your sender reputation through consistent, good sending practices is the most effective way to ensure timely delivery.
- Message content and pre-processing: Emails with complex dynamic content or large image sizes require more processing time before being handed off to the MTA, which can affect the overall queue-to-delivery speed. This is mentioned as a factor in optimizing email performance.
- ESP infrastructure and configuration: While MTAs are fast, the ESP's overall architecture, network capabilities, and how they configure their sending queues and connections significantly impact the real-world sending rates.
What email marketers say
Email marketers often seek clear benchmarks for ESP sending speeds to manage client expectations and campaign timing. However, many find that raw speed metrics provided by ESPs during sales cycles can be deceptive. The practical experience of marketers suggests that actual delivery times are less about the ESP's theoretical maximum throughput and more about the interplay of sender reputation, mailbox provider reception policies, and the complexity of the email content itself. Variations in delivery speed between different ESPs for the same list size are commonly observed, highlighting the nuanced nature of email throughput.
Key opinions
- Actual vs. theoretical speed: Marketers recognize that the hypothetical maximum sending rate advertised by ESPs often doesn't align with the real-world time it takes for emails to be delivered to recipients' inboxes, especially due to ISP throttling.
- Reputation over throughput: The general consensus is that a slow sending rate (e.g., 100K emails in 12 hours) is a strong indicator of a reputation issue rather than an ESP's technical limitation.
- Queue time vs. delivery time: There's a distinction between how fast an ESP can queue emails for delivery internally versus how quickly those emails are actually accepted and delivered by mailbox providers.
- Impact of content and pre-processing: Dynamic content within email templates can add significant pre-processing time, affecting the overall speed before emails even reach the outbound MTA queue.
Key considerations
- Client expectations: ESPs often face client questions about how long it will take to send a campaign of a specific size, making it important to manage expectations based on real-world deliverability factors, not just raw sending capacity.
- Vendor claims scrutiny: Marketers should be wary of sales cycle claims regarding high sending speeds, as these numbers are often theoretical maximums under ideal conditions and may not reflect typical campaign performance.
- Variability between ESPs: Experiences show significant differences in actual sending times between various ESPs, even for similar campaign volumes, indicating that ESP configuration and practices play a role.
- Internal processing time: The time it takes for an ESP to process dynamic content and prepare emails for sending can significantly affect the overall time from initiating a send to actual delivery, independent of the network transmission speed.
Marketer from Email Geeks indicates that receiving Mailbox Provider (MBP) specific average send rates per hour would be incredibly useful. This would need to consider the number of IPs in the sending mix to be accurate.
Email marketer from Joe PR suggests that email marketing performance benchmarks, including open rates and click-through rates, are more indicative of success than raw sending speed, guiding optimization efforts.
What the experts say
Experts in email deliverability clarify that while Message Transfer Agents (MTAs) and ESP infrastructure possess high sending capacities, actual email delivery speed is predominantly dictated by the receiving Mailbox Providers (MBPs) and the sender's reputation. They underscore that ESPs intentionally manage sending rates to align with ISP connection limits and avoid triggering spam filters. The focus for high-volume senders should thus be on maintaining stellar sender hygiene and optimizing content, rather than solely on raw throughput figures which are often misleading.
Key opinions
- ISP control: The receiving ISPs primarily impose email sending limits, and these are based on how recipients interact with the mail from a particular sender. This is a crucial distinction from an ESP's internal capacity.
- Reputation-driven delays: If email delivery is exceptionally slow, it is almost always a direct result of a poor sender reputation, leading to mailbox providers throttling or refusing mail.
- Authentication impact: While DKIM signing involves CPU consumption, experts note that the overhead from cryptographic operations is generally negligible compared to network costs and ISP throttling. Proper email authentication is critical for speed.
- Shared vs. dedicated IPs: While using more IPs can help deliver faster, the benefit is often offset if those IPs are shared and thus their reputation is influenced by other senders on the same pool.
Key considerations
- Beyond raw MTA speed: Mail Transfer Agents (MTAs) can send mail extremely fast, but this capacity is often limited by the receiving mailbox providers, not the ESP's technical infrastructure, unless the ESP's setup is suboptimal for the sender's needs.
- Network management complexity: Effective TCP network management, including handling high-volume TLS connections, is crucial for maintaining optimal sending speeds and avoiding transient errors that can cause delays.
- Lack of public benchmarks: There are no widely published, normalized benchmarks for ESP sending speeds because the actual performance is so heavily dependent on individual message characteristics and sender reputation. Multiple factors influence deliverability.
- Problem diagnosis: Significant delays might indicate underlying problems beyond just reputation, such as issues with older MTA versions that struggle with high-volume TLS, leading to network resets and slowdowns.
Expert from Email Geeks indicates that most ESP sending speed is intentionally limited by design. They follow best practices on how many emails to send to specific Mailbox Providers per hour or connection.
Expert from SpamResource highlights that email sender reputation is paramount. If reputation is low, ESPs are less likely to deliver emails to the inbox, often resulting in them landing in spam or being blocked.
What the documentation says
Official documentation and research often emphasize the technical foundations and best practices that govern email sending speeds. These sources consistently highlight the role of authentication protocols (SPF, DKIM, DMARC), IP reputation, and adherence to industry standards in ensuring efficient and reliable email delivery. While they may not provide explicit benchmarks for ESP sending speeds, they detail the mechanisms by which mailbox providers manage inbound email traffic, including throttling and connection limits. The underlying message is that speed is an outcome of good deliverability practices, not an isolated metric.
Key findings
- Authentication's role: Domain authentication mechanisms like SPF, DKIM, and DMARC are fundamental to building trust with mailbox providers, directly influencing how quickly and reliably emails are accepted, as highlighted by email analysis guides.
- Sender reputation metrics: Mailbox providers track various metrics (e.g., spam complaints, bounces, engagement) to form a sender's reputation, which directly dictates sending limits and acceptance rates.
- ISP-level configurations: Mailbox providers employ sophisticated systems to manage incoming email traffic, including dynamic throttling based on sender reputation and network capacity, rather than fixed, high-speed benchmarks.
- SMTP standards adherence: ESPs must adhere to SMTP sending standards, which includes handling transient errors and back-off strategies, further influencing the effective sending speed to avoid overloading receiving servers.
Key considerations
- Impact of IP reputation: A good IP reputation is crucial for avoiding throttling. ESPs with well-managed IP warming processes and robust reputation management are essential for consistent high-speed delivery.
- Message size and complexity: Larger email sizes and more complex HTML/dynamic content can naturally slow down transmission and processing times, both at the ESP and the receiving end.
- Network latency and server location: The physical distance between the sending ESP's servers and the receiving Mailbox Provider's servers, as well as general network latency, can affect the speed of delivery.
- List quality: Sending to a clean, engaged list minimizes bounces and spam complaints, which in turn preserves sender reputation and allows for higher sending rates without triggering ISP throttling. This is a common theme in email reputation strategies.
Documentation from Customer.io indicates that any high-quality ESP should enforce proper domain authentication, comply with SMTP sending standards, and maintain specific ISP-level configurations to ensure reliable deliverability.
Documentation from Encharge.io explains that email reputation is influenced by various factors, including the sender's authentication setup, historical sending practices, and the overall quality of the email content.
