Why do email rate limit bounces drop significantly after implementing sending throttling with HubSpot?
Matthew Whittaker
Co-founder & CTO, Suped
Published 22 Jul 2025
Updated 18 Aug 2025
7 min read
When you notice a significant drop in email rate limit bounces after implementing sending throttling, especially with a platform like HubSpot, it can feel a bit too good to be true. I've seen situations where a client on a dedicated IP, sending to 400k subscribers, was hitting 5-6% rate limit bounces. After introducing a custom throttling mechanism, distributing the send across 20 groups with 10-minute delays, the bounce rate plummeted to 0.04%.
This dramatic reduction might seem suspicious at first glance, but it's a very real and expected outcome of proper email throttling. The core reason lies in how mailbox providers, like Gmail, manage incoming email volume and their sophisticated anti-spam defenses. When you send emails too quickly, especially in large volumes, it can trigger rate limiting mechanisms.
Why rate limit bounces occur
Mailbox providers implement rate limits to protect their systems and users from spam and abuse. If an IP address or domain sends emails at an abnormally high rate in a short period, it can be flagged as suspicious. This doesn't necessarily mean you're sending spam, but it signals to the receiving server that something unusual is happening, leading to temporary blocks or rate limit bounces.
These bounces aren't typically due to a non-existent email address, but rather a temporary refusal to accept more mail at that moment. Many email service providers (ESPs) like HubSpot have their own default throttling mechanisms, but they might not always be aggressive enough for specific sending patterns or large lists, especially if your sender reputation is still building or recovering. When HubSpot registers a rate limit, it may interpret it as a soft bounce if the issue is temporary, or sometimes even as a hard bounce depending on their internal handling logic, which can disproportionately affect your listed bounce rates.
By manually introducing a slower, more staggered sending pattern, you are effectively respecting the mailbox providers' implicit or explicit rate limits. Instead of barraging their servers with hundreds of thousands of emails at once, you're delivering them in manageable batches over a longer period. This staggering of email sends mimics the natural sending behavior of a trusted sender and helps maintain a positive relationship with receiving servers. It signals that your sending is legitimate and not a sudden, potentially malicious, burst.
Understanding HubSpot's bounce reporting
HubSpot tracks various types of bounces, including soft bounces for temporary issues and hard bounces for permanent ones. When rate limits occur, HubSpot might categorize them in a way that contributes to your bounce rate, even if the issue is transient. Your custom throttling prevents these temporary rejections from even happening, so HubSpot sees fewer 'bounce' events.
The impact of throttling on reputation and deliverability
The primary impact of throttling is on your sender reputation. Mailbox providers continuously assess your sending behavior. High bounce rates, even if they are soft bounces due to rate limiting, can negatively affect how your sending IP and domain are perceived. By drastically reducing these bounces, you are actively improving your reputation with recipients like Gmail.
This leads to fewer emails being deferred or blocked, and a higher proportion of your emails successfully reaching the inbox. Think of it as building trust. When you send email responsibly and don't overwhelm recipient servers, they are more likely to accept your mail. This is crucial for long-term email deliverability and inbox placement.
While your custom throttling is effective, it also highlights potential underlying issues. Sometimes, if an ESP's default throttling isn't sufficient for your volume, it could be a sign that your list health isn't optimal, or that your previous sending practices (like cold outbound) have impacted your reputation. The solution you've implemented compensates for this by providing a gentler ramp-up of sends.
Example of mail server throttling commandsBASH
mail send throttle <group> <limit>
mail delivery throttle <group> <limit>
HubSpot's role and sender reputation
HubSpot, like other large ESPs, manages a vast number of sending IPs and users. While they do implement email throttling to maintain good deliverability across their platform, their default settings might be generalized. For senders with very large lists, particularly those transitioning from other platforms or with historical reputation challenges, these default settings may not be granular enough.
It's possible that your dedicated IP, even with HubSpot's general throttling, was still sending too rapidly for certain mailbox providers. Your custom throttler, by introducing longer delays and smaller batches, aligns your sending behavior more closely with what major inbox providers expect for optimal deliverability. This often involves reducing the peak sending rate to prevent triggering blocklist entries or temporary blocks. While HubSpot might treat certain bounce types as deferrals that are retried, a higher volume of these deferrals can still reflect negatively on your sender score.
Furthermore, if you've recently ceased cold outbound activities through HubSpot, it takes time for your IP and domain reputation to recover. The stricter throttling helps to mitigate the impact of past activities and demonstrate a consistent pattern of good sending behavior. This proactive approach helps build a stronger relationship with inbox providers over time.
Risk to reputation: Potential negative impact on IP and domain sender reputation with mailbox providers.
Sub-optimal deliverability: Emails more likely to be deferred or sent to spam folders.
After throttling
Dramatic reduction in bounces: Bounces drop to 0.04%, indicating successful rate limit adherence.
Improved sender reputation: Demonstrates responsible sending, enhancing trust with receivers.
Enhanced inbox placement: Higher likelihood of emails reaching the primary inbox due to better behavior.
Best practices for sustained deliverability
While your manual throttling has yielded excellent results, it's worth investigating the precise bounce messages you're receiving. HubSpot's categorization of bounce reasons might sometimes simplify a more complex underlying issue. For instance, a Rate Limit bounce from Gmail could imply low IP reputation, a sudden volume jump, or suspicion of unsolicited mail, rather than just hitting a raw numerical sending limit.
Even with successful throttling, continuous monitoring of your Postmaster Tools (if sending to Gmail addresses) and other deliverability metrics remains essential. This will help you understand if there are any subtle shifts in how your emails are being received, beyond just the bounce rate. For instance, are your open rates consistent, and is your mail landing in the primary inbox?
Your solution is a testament to the power of understanding how mailbox providers operate and adapting your sending strategy accordingly. By aligning your email cadence with their expectations, you can significantly reduce bounces and improve your overall deliverability, even with large sending volumes. This proactive management is key to sustained email success.
Continuous deliverability improvement
Even after seeing positive results from throttling, remember that email deliverability is an ongoing process. Maintain a clean list, monitor engagement, and keep an eye on your sender reputation. Throttling is a powerful tool, but it's part of a larger deliverability strategy that includes content quality, audience engagement, and proper authentication.
Conclusion: the power of proper throttling
The significant drop in rate limit bounces after implementing sending throttling with HubSpot is a direct result of adapting your sending behavior to the expectations of mailbox providers. It's a clear indication that previous sending volumes, even with HubSpot's default mechanisms, were too aggressive for your specific sender profile and list. By manually staggering your sends, you've improved your standing with recipients, reduced temporary rejections, and consequently, lowered your reported bounce rate.
This optimization not only decreases bounces but also helps build a stronger sender reputation over time, leading to better inbox placement and overall email program success. It's a smart, proactive step towards healthier email deliverability.
Views from the trenches
Best practices
Always segment large email lists into smaller batches for sending to reduce the likelihood of triggering rate limits and improve overall deliverability.
Utilize gradual sending over several hours or days for very large campaigns to allow mailbox providers to process emails without overwhelming their systems.
Monitor your domain and IP reputation using tools like Google Postmaster Tools to identify any potential issues before they lead to severe throttling or blacklisting.
Regularly clean your email lists to remove inactive or invalid addresses, reducing bounce rates and protecting your sender reputation.
Ensure your email authentication records (SPF, DKIM, DMARC) are correctly configured to prove legitimacy to receiving mail servers.
Common pitfalls
Sending large volumes of emails to an unengaged or old list can quickly lead to high bounce rates and trigger rate limiting by mailbox providers.
Relying solely on an ESP's default throttling settings without understanding your specific sending patterns and recipient server responses can be insufficient.
Ignoring 'rate limit' bounce messages or misinterpreting them as hard bounces can lead to unnecessary contact suppression and missed delivery opportunities.
Failing to adapt sending speeds after changes in email content, list quality, or sender reputation, leading to continued deliverability issues.
Not elevating significant, persistent rate limit issues to your ESP's support team for deeper investigation into the specific bounce codes and their root causes.
Expert tips
Consider that high rate limit bounces from a specific provider, like Gmail, might indicate a low sending IP reputation rather than just a volumetric limit.
Be aware that some ESPs might classify temporary deferrals (throttles) as soft bounces, artificially inflating your bounce rate even if retries would succeed.
If migrating platforms, new email providers may place you in an IP pool with lower connection limits, necessitating more aggressive self-throttling.
Always seek the full, unredacted bounce messages from your ESP, as simplified internal reporting can obscure the true reason for a delivery failure.
A sudden drop in bounces after throttling often confirms that the previous issue was indeed a rate limit or a technical connection timeout, not necessarily an SMTP error.
Expert view
Expert from Email Geeks says that understanding which provider is throttling and the exact bounce codes is crucial, as HubSpot might sometimes mislabel deferrals as soft bounces.
2025-06-13 - Email Geeks
Marketer view
Marketer from Email Geeks says that they are suspicious of their bounce reports, especially since they've stopped cold outbound, which should improve IP reputation.
