What are acceptable email sending speeds and why do ESPs throttle or pause email delivery?
Michael Ko
Co-founder & CEO, Suped
Published 20 Jul 2025
Updated 19 Aug 2025
8 min read
When managing email campaigns, a common question arises: what are acceptable email sending speeds? Many believe faster is always better, but the reality is more nuanced. Email service providers (ESPs) and mailbox providers (ISPs) often throttle or pause email delivery, which might seem counterproductive, but it's a critical function for maintaining a healthy email ecosystem. It is not about how fast you can send, but rather how fast recipient ISPs will accept your emails without triggering negative responses.
Understanding these dynamics is crucial for ensuring your emails reliably reach the inbox rather than being delayed, bounced, or even sent to spam. Effective email deliverability is a delicate balance of speed, volume, and sender reputation, all orchestrated to align with the expectations and limitations of mailbox providers.
Understanding email throttling and rate limits
Email throttling is a mechanism where Internet Service Providers (ISPs) intentionally limit the number of emails a sender can send within a specific period. This practice is primarily designed to protect their networks and users from spam, abusive sending, and potential overload. When you send emails, your ESP interacts with various ISPs, and each ISP has its own set of rate limits. These limits can vary widely based on your sending reputation, the volume of mail, and even the time of day.
ESPs manage these rate limits by dynamically adjusting sending speeds. If an ESP attempts to send too many emails too quickly to a particular ISP, the ISP might respond with temporary errors (deferrals) or, in more severe cases, start rejecting emails or blocklisting the sending IP or domain. An effective ESP will interpret these signals and automatically slow down (throttle) or pause sending to avoid reputation damage and ensure eventual delivery.
The goal of throttling is to maintain a positive sender reputation and avoid being flagged as a spammer. A good ESP implements sophisticated algorithms to manage these complexities, ensuring that even if a campaign takes a bit longer, it ultimately achieves high inbox placement. This proactive management is crucial for the long-term health of your email program.
The ESP's role in throttling
Your ESP acts as a crucial intermediary, adapting your sending speed based on real-time feedback from various mailbox providers. This prevents you from inadvertently triggering spam filters or damaging your sender reputation. It is a protective measure designed to ensure your long-term deliverability.
Why ESPs throttle or pause delivery
ESPs and ISPs throttle or pause delivery for several interconnected reasons, primarily centered around maintaining a healthy and reliable email ecosystem. The most significant factor is sender reputation. Mailbox providers carefully monitor incoming email streams. If they detect a sudden spike in volume from an unfamiliar or low-reputation sender, or if they observe negative signals like high bounce rates, spam complaints, or low engagement, they will immediately begin to throttle or defer messages.
Another reason is to prevent network abuse and resource exhaustion. Uncontrolled sending could overwhelm an ISP's servers, leading to service degradation for all users. By implementing rate limits, ISPs can ensure fair access to their systems and prevent malicious actors from flooding their networks. Furthermore, sophisticated email filters and blacklists rely on observing sending patterns over time to accurately assess legitimacy. Throttling gives these systems the necessary time to analyze behavior and make informed decisions about incoming mail.
Ultimately, when an ESP pauses or slows delivery, it is often a strategic move based on real-time feedback from ISPs. This ensures that even if a campaign takes longer to complete, it stands a much better chance of reaching the recipient's inbox rather than being rejected outright. It is a sign of a responsible ESP protecting your sending infrastructure and reputation, even if it causes a temporary inconvenience in speed.
Reasons for throttling
Reputation management: ISPs limit volume to new or low-reputation senders, or those with poor engagement metrics, to prevent spam. This includes monitoring for high bounce rates and spam complaints.
System overload prevention: Prevents networks from being overwhelmed by large, sudden influxes of mail, ensuring stability for all users.
Content analysis: Allows ISPs time to scan email content for malicious links, viruses, or spam characteristics before delivery.
Impact of ignoring throttling
Blacklisting: Aggressive sending can lead to your IPs or domains being added to email blocklists or blacklists, preventing future delivery.
Permanent rejections: Beyond deferrals, emails may be permanently rejected, resulting in lost outreach and wasted resources.
Decreased deliverability: Even if not blocked, your emails may experience severe delays or land in spam folders, impacting campaign effectiveness.
Acceptable sending speeds and metrics
There is no universal acceptable email sending speed that applies to all senders and all scenarios. What's considered fast for one sender might be too fast for another. The optimal sending speed is highly dynamic and depends on numerous factors, primarily your sender reputation, the specific mailbox provider you're sending to, and the historical volume and engagement from your domain. For instance, sending at 480 emails per minute might seem slow compared to 25,000 per minute, but if it ensures inbox placement, it's the more effective speed.
ISPs like Gmail, Outlook, and Yahoo Mail have sophisticated systems that adjust their acceptance rates based on your domain's reputation and IP history. A sender with a long history of good engagement, low complaints, and consistent volume will generally be able to send emails faster than a new sender or one with fluctuating sending patterns. This is why staggering email sends and gradually increasing volume (IP warming) are crucial practices.
The true measure of acceptable sending speed isn't raw throughput, but successful inbox placement. As Twilio's blog highlights, if you try to send above the acceptable threshold, ISPs will reject your email, leading to higher bounces. A delay is far better than a rejection or, worse, landing in the spam folder. Focus on a speed that ISPs are happy to receive, which aligns with their filtering and anti-abuse policies.
ISP
Typical considerations
Notes
Gmail
High volume for trusted senders, lower for new or unauthenticated sources.
Focuses heavily on user engagement and sender reputation.
Outlook/Microsoft
Strict rate limits, particularly for new IPs. Focus on slow, consistent ramp-up.
Requires consistent good behavior to build trust.
Yahoo/AOL
Generally high limits for established, trusted senders. Responsive to real-time feedback.
Pays close attention to complaint rates.
Strategies to optimize email sending speed
To achieve optimal email sending speeds and avoid throttling, a multi-faceted approach is necessary. First, prioritize list hygiene. Regularly clean your email lists to remove inactive, invalid, or problematic addresses. Sending to a clean, engaged list is the single most important factor in maintaining a positive sender reputation. Also, segment your audience and manage sending volume spikes strategically, especially for large campaigns. Instead of blasting all emails at once, distribute them over a reasonable period.
Second, focus on engagement. ISPs observe how recipients interact with your emails, including opens, clicks, replies, and whether they mark your email as spam. High engagement signals to ISPs that your emails are valuable, earning you more trust and, consequently, greater sending leniency. Conversely, low engagement or high complaint rates will trigger throttling and potentially lead to blocklisting. Encourage recipients to add you to their address book.
Finally, ensure robust email authentication. Properly configured SPF, DKIM, and DMARC records verify your sending identity and build trust with ISPs. This reduces the likelihood of your emails being flagged as suspicious, even at higher volumes. Combining these strategies creates a strong foundation for optimizing your email sending speed and ensuring reliable deliverability.
Tips for improving sending speed
Start slowly: Especially with new IPs or domains, gradually increase your sending volume to build trust.
Monitor deliverability metrics: Keep an eye on bounces, complaints, and engagement rates to adjust your strategy.
Segment campaigns: Split large sends into smaller batches, especially for less engaged segments.
Engage recipients: Send relevant content to encourage positive interactions and reduce spam complaints.
Monitor your engagement metrics closely; high engagement is key to ISP trust.
Segment your audience and send smaller, targeted batches to specific ISPs to manage rate limits.
Proactively encourage subscribers to add your sending address to their contact list.
Maintain pristine list hygiene to avoid sending to invalid or disengaged email addresses.
Common pitfalls
Expecting instant delivery for large volumes, as ISPs prefer slower, consistent sending.
Ignoring SMTP error codes from ISPs, which indicate throttling or blocklisting.
Sending to unengaged segments or old lists, leading to higher spam complaints.
Over-relying on a single metric like SenderScore, which isn't always predictive.
Expert tips
If you have urgent, time-sensitive emails, consider using a separate ESP account or dedicated IP.
ISP tolerance for emails is increasingly influenced by domain/sender reputation, with IP reputation becoming less dominant.
Any decent mail server should delay or slow traffic upon receiving specific SMTP errors from the receiving server.
Unoptimized throttling logic can cause sending to halt entirely after hitting specific hourly thresholds.
Marketer view
A marketer from Email Geeks says that their vendor's email sending speed has significantly decreased from 25,000 to 480 emails per minute, leading to much longer delivery times for large campaigns.
2019-06-26 - Email Geeks
Marketer view
A marketer from Email Geeks says that the critical factor in email delivery isn't how fast a sender can send mail, but rather how quickly recipient ISPs are willing to accept it.
2019-06-26 - Email Geeks
Maintaining optimal email delivery
Achieving optimal email delivery is less about raw sending speed and more about strategic pacing that respects ISP limits and protects your sender reputation. While it might seem counterintuitive for an ESP to throttle or pause your emails, these actions are often necessary to ensure that your messages reach the inbox successfully in the long run. Aggressive, unmanaged sending can quickly lead to blocklists (or blacklists) and permanent damage to your domain's credibility.
By understanding the reasons behind throttling and implementing best practices like list hygiene, engagement monitoring, and proper authentication, you can work with your ESP to navigate the complexities of email delivery. This approach ensures your email program is not only fast enough but also consistently reliable, maximizing your reach and impact.
Gmail, 
Outlook, and 
Yahoo Mail have sophisticated systems that adjust their acceptance rates based on your domain's reputation and IP history. A sender with a long history of good engagement, low complaints, and consistent volume will generally be able to send emails faster than a new sender or one with fluctuating sending patterns. This is why staggering email sends and gradually increasing volume (IP warming) are crucial practices.
Gmail
Outlook/Microsoft
Yahoo/AOL
