Summary
Implementing sending throttling, especially when integrated with platforms like HubSpot, dramatically reduces email rate limit bounces. This practice effectively manages email delivery speed, ensuring messages are sent at a pace compatible with recipient server capacities. By preventing the overwhelming of mail servers, throttling avoids the temporary rejections, typically 4xx SMTP errors, that occur when senders transmit excessive volume within a short timeframe, thereby also fostering a stronger sender reputation and bypassing automated spam defenses.
Key findings
- Pacing Email Volume: Throttling controls the rate of email delivery, preventing senders from exceeding recipient server volume thresholds and connection limits, which are common triggers for 4xx temporary rejections.
- Enhancing Sender Reputation: Sending emails at a measured, non-aggressive speed signals responsible behavior to mailbox providers, building trust and improving sender reputation, which reduces the likelihood of being perceived as spam-like.
- Preventing Server Overload: This strategy avoids overwhelming recipient mail servers with a sudden influx of messages, thereby preventing temporary blocks or rejections designed to protect their infrastructure from excessive incoming mail.
- Adaptive Sending Strategy: Throttling enables an adaptive approach, effectively backing off and sending emails in smaller batches when encountering resistance, ensuring the sending rate remains within acceptable parameters for recipient domains.
Key considerations
- HubSpot's Built-in Features: HubSpot's email infrastructure often includes inherent rate management. Users 'implementing throttling' may be leveraging or aligning with these built-in mechanisms that intelligently pace email sends to optimize deliverability.
- Diverse Bounce Causes: Not all reported 'rate limit' bounces are direct SMTP rejections; some could stem from internal HubSpot technical errors like Java timeouts, or be indicative of issues with assigned IP pool reputation, warranting direct support engagement for clarity.
- Underlying Deliverability Factors: True rate limits from major providers like Gmail are frequently tied to factors such as low sender reputation, significant jumps in sending volume, or high spam complaint rates, suggesting throttling is one component of a broader deliverability strategy.
- MTA vs. ISP Throttling: The perceived throttling might originate from the client's Mail Transfer Agent (MTA) and its assigned IP pool's connection limits, rather than directly from the recipient ISP, particularly if HubSpot's system is configured to drop messages after hitting a throttle limit instead of retrying.
What email marketers say
12 marketer opinions
The significant drop in email rate limit bounces observed after implementing sending throttling, whether user-configured or managed by HubSpot's internal systems, stems from a disciplined approach to email delivery. This method ensures that messages are dispatched at a pace that aligns with recipient mail server capacities and their unstated volume thresholds. By preventing the overwhelming of mail servers, throttling avoids the temporary rejections, often characterized as 4xx SMTP errors, which are common responses to excessive sending. Furthermore, this controlled sending strategy reinforces a positive sender reputation and adeptly navigates automated spam defenses.
Key opinions
- Respecting Server Capacity: Throttling precisely manages sending speed to align with recipient server limits, both stated and unstated (per hour or per day), preventing temporary rejections that occur when these capacities are exceeded.
- Enhancing Sender Trust: By distributing email volume over time, throttling signals responsible sending behavior to mailbox providers, building a positive sender reputation and avoiding triggers for automated spam defenses and temporary blocks.
- Preventing Overwhelm: This strategic pacing actively prevents sending IPs from appearing aggressive or overwhelming recipient mail server infrastructure, which is a direct cause of SMTP 4xx errors like 421 rate limit bounces.
- Adaptive Delivery: Throttling enables an adaptive sending strategy, where the system effectively 'backs off' or slows down when encountering resistance, ensuring continuous delivery within acceptable volume parameters for diverse domains.
Key considerations
- HubSpot's Internal Mechanisms: HubSpot's email infrastructure often includes native rate management. User-implemented throttling may align with or enhance these built-in systems designed to intelligently pace email sends.
- Differentiating Bounce Types: Not all reported 'rate limit bounces' are direct SMTP rejections; some could be internal platform issues, for example Java timeouts, or tied to specific IP pool limitations, necessitating deep investigation with HubSpot support.
- Holistic Deliverability View: While throttling is effective, actual rate limits from major mailbox providers are frequently indicative of broader issues such as low sender reputation, sudden volume increases, or elevated spam complaint rates, requiring a comprehensive deliverability strategy.
- MTA and Retry Logic: Perceived throttling might originate from the client's Mail Transfer Agent (MTA) or its assigned IP pool's connection limits, with HubSpot potentially dropping messages after a throttle limit instead of consistent retries, emphasizing the need for support clarification.
Marketer view
Email marketer from Email Geeks explains that HubSpot might be soft bouncing what should be deferrals and that actual Gmail rate limit bounces typically relate to low reputation, large volume jumps, or high spam complaints. He advises elevating the issue to HubSpot support to get the real bounce errors and suggests that the reported rate issues are usually due to reputation, technical connections, or list health.
14 Jan 2025 - Email Geeks
Marketer view
Email marketer from Email Geeks explains that the client's Mail Transfer Agent (MTA) is likely throttling connections, not the ISP, possibly due to being in an IP pool with lower connection limits. He also notes that the specific bounce message reported isn't provided by Gmail and suggests that HubSpot's system might be configured to drop messages after hitting a throttle limit instead of retrying, recommending raising a ticket with HubSpot.
2 Feb 2025 - Email Geeks
What the experts say
3 expert opinions
The substantial decrease in email rate limit bounces after implementing sending throttling, whether through user configuration or HubSpot's inherent systems, directly addresses a common issue: sending emails faster than recipient mail servers can process them. This strategic pacing prevents temporary rejections by aligning the send rate with ISP acceptance capacities. While throttling effectively mitigates these bounces, it is also important to consider that some reported 'rate limit' issues might stem from HubSpot's internal technical errors or the reputation of assigned IP pools, not always direct SMTP rejections.
Key opinions
- Speed Matching: Throttling ensures the email transmission speed aligns with the recipient ISP's acceptance capacity, directly preventing temporary rejections that occur when volumes are too high.
- Bounce Prevention: By regulating the sending pace, throttling directly reduces the occurrence of rate limit bounces, which are temporary rejections by mail servers designed to manage incoming message flow.
- ISP Alignment: This method aligns the sender's email flow with the stated and unstated limits of recipient mail servers, thereby avoiding overwhelming their infrastructure and triggering automated defenses.
Key considerations
- HubSpot IP Reputation: The reputation of IP pools HubSpot assigns to clients can significantly influence whether rate limits are encountered, suggesting a potential underlying cause for bounces.
- Internal System Errors: Some reported 'rate limit bounces' might actually be internal technical errors within HubSpot, such as Java timeouts, rather than actual SMTP rejections from recipient mail servers.
- Bounce Message Accuracy: It is crucial to verify if a reported bounce message is a genuine SMTP rate limit rejection or an internal platform error that HubSpot might be mislabeling, necessitating deeper investigation.
Expert view
Expert from Email Geeks explains that HubSpot might assign clients to IP pools with poor reputations, leading to rate limits. She also critically points out that the reported bounce message might not be an SMTP bounce at all but rather an internal technical error (e.g., Java timeout) within HubSpot, suggesting the problem might not even be an SMTP deliverability issue.
1 Oct 2021 - Email Geeks
Expert view
Expert from Word to the Wise explains that email rate limit bounces happen when senders transmit messages faster than recipient ISPs are prepared to accept. By implementing sending throttling, senders intentionally slow down their sending pace to align with the ISP's acceptance limits, thereby preventing these temporary rejections and causing a significant drop in rate limit bounces.
10 Nov 2021 - Word to the Wise
What the documentation says
5 technical articles
Implementing sending throttling, whether directly managed by a user or facilitated by HubSpot's inherent systems, dramatically reduces email rate limit bounces by proactively respecting recipient server limitations. This strategy prevents the mail servers from being overwhelmed, thereby avoiding the temporary 4xx SMTP rejections that signal excessive sending. By appearing as a cooperative and legitimate sender, rather than a source of high-volume traffic that might trigger defensive blocks, throttling ensures a smoother, more consistent email flow acceptable to most mailbox providers.
Key findings
- Recipient Server Defenses: Recipient mail servers implement rate limits as a core defense mechanism against overwhelming traffic, protecting resources and preventing abuse by issuing temporary 4xx SMTP rejections when volume thresholds are exceeded.
- Controlled Traffic Flow: Throttling ensures email traffic is distributed over time, maintaining a consistent and acceptable sending pace that prevents sudden, large influxes of mail which commonly trigger temporary blocks and rate limit bounces.
- Cultivating Trust: By sending emails at a moderated rate, senders project an image of legitimacy and cooperation, which helps build and maintain a positive sender reputation with mailbox providers, thereby reducing the likelihood of rate-based rejections.
Key considerations
- Platform Integration: HubSpot's system inherently manages email sending rates, meaning user-initiated throttling often synergizes with or leverages these built-in deliverability features to optimize mail flow.
- Transient Error Nature: Rate limit bounces, typically represented by 4xx SMTP codes, are temporary rejections, indicating that while the server is currently overwhelmed or suspicious, it may accept the email later if the sending rate is adjusted.
- Comprehensive Deliverability: While effective, throttling is one component of a broader deliverability strategy; sustained rate limits from major providers can also stem from factors such as low sender reputation, sudden volume increases, or high spam complaints.
Technical article
Documentation from Postmark explains that rate limit bounces, often seen as 4xx SMTP errors (e.g., 421, 451), are temporary rejections by recipient servers when a sender attempts to send too many emails within a short timeframe. Implementing sending throttling significantly reduces these bounces by pacing out email delivery, which allows recipient servers to process mail without being overwhelmed, thereby avoiding triggers for temporary blocks or rejections due to perceived spamming or high volume.
1 May 2023 - Postmark Blog
Technical article
Documentation from Mailgun shares that recipient domains enforce rate limits to protect against abuse and server overload. When an email sending system, like one integrated with HubSpot, implements sending throttling, it deliberately slows down the rate at which emails are sent to a specific domain or IP address. This practice prevents the sender from hitting the recipient's volume thresholds, thus reducing the occurrence of temporary rate limit bounces (e.g., SMTP 421 errors) by appearing as a cooperative sender.
12 Jul 2021 - Mailgun
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