Why is my email getting throttled by Gmail and Microsoft despite high engagement and good reputation?
Matthew Whittaker
Co-founder & CTO, Suped
Published 22 Apr 2025
Updated 15 Aug 2025
7 min read
Email throttling can be incredibly frustrating, especially when you feel like you are doing everything right. Your engagement rates are high, your sender reputation looks solid, yet your emails are still getting held up, sometimes for significant periods, by major mailbox providers like Gmail and Microsoft. It feels like a paradox, doesn't it? This situation is more common than you might think and points to a deeper, more nuanced understanding of how these providers evaluate incoming mail.
The core of the issue often lies beyond the superficial metrics, touching on subtle shifts in sending patterns, underlying list quality issues, or even evolving policies from the mailbox providers themselves. We will explore why this phenomenon occurs and what steps you can take to regain smooth email delivery.
Understanding email throttling's nature
Email throttling, also known as rate limiting, is a mechanism Internet Service Providers (ISPs) use to control the flow of incoming mail. It limits the number of emails an IP address or domain can send to their recipients within a specific timeframe. This isn't inherently malicious, it's a defense mechanism to protect their users from spam, maintain server stability, and mitigate potential abuse.
Even with high engagement and a good reputation, a sudden or unusual surge in email volume, or unexpected patterns, can trigger these protective measures. ISPs (email service providers) act as gatekeepers, and any perceived anomaly, even from a typically good sender, can lead to temporary restrictions as they analyze the traffic. Think of it like a security checkpoint, even a regular traveler might be subject to extra screening if their behavior deviates slightly from the norm.
Beyond the visible metrics you see in Postmaster Tools or other sender reputation dashboards, ISPs employ complex algorithms to assess your sending behavior. A key factor can be the spam rate threshold each provider maintains. Even small, incremental increases in complaint rates or spam trap hits, which might not immediately tank your overall score, can trigger throttling. These subtle signals, combined with perceived irregular mail traffic, are often the culprits.
It's also crucial to consider your list quality. Even with high engagement overall, sending to a small segment of unengaged or problematic recipients, or inadvertently hitting spam traps, can disproportionately affect your sending reputation. ISPs prioritize protecting their users, and a few negative signals can outweigh many positive ones. You can learn more about email sender reputation from Mailjet.
Authentication standards are also becoming stricter. While you might have SPF, DKIM, and DMARC set up, any misconfiguration or failure in these records can lead to emails being throttled or outright rejected. Mailbox providers are increasingly stringent on these foundational elements of email security. Check out our simple guide to DMARC, SPF, and DKIM for more.
Common sender beliefs
High open rates: Guarantees inbox delivery because users are engaging.
Dedicated IP: Sufficient for any send volume without issues.
Consistent volume: No changes mean no new problems should arise.
ISP realities
Engagement context: ISPs weigh negative signals (complaints, traps) heavily, even amidst high engagement. They look at engagement per recipient, not just overall.
IP reputation nuances: While a dedicated IP provides control, it's still subject to strict ISP policies and potential blocklisting (or blacklisting).
Traffic patterns: Breaking up sends, or any unannounced change in volume or cadence, can be flagged as 'irregular' by automated systems.
ISP specific nuances for Gmail and Microsoft
Gmail is highly sophisticated in its filtering. While Google Postmaster Tools provides valuable insights into your domain and Gmail IP reputation, it doesn't show the full picture. Gmail's algorithms weigh individual recipient engagement, spam complaints, direct spambox moves, and the number of unknown users on your list. A sudden increase in volume, even if split into batches, can appear as an anomaly if it deviates from your established historical sending patterns. This can trigger temporary throttling as Gmail assesses the change.
Microsoft (Hotmail, Outlook.com, Microsoft 365) has also tightened its filtering recently. Their concern with irregular mail traffic is a strong indicator of their sensitivity to changes in sending behavior, regardless of existing good reputation. They specifically mention server/network compromise or compromised user accounts as potential reasons for sudden changes in email sending volume. Even breaking up a large send into smaller batches over different days can be interpreted as an anomaly if it deviates from their learned sending profile for your IP.
For Microsoft, it’s critical to monitor your Sender Network Data Services (SNDS) and ensure you are in compliance with their latest sender requirements, particularly for high-volume senders. Any deliverability issues with Microsoft often require direct engagement with their postmaster team, backed by data.
Key differences in ISP throttling
Gmail's dynamic throttling: Highly adaptive, responding to real-time engagement, individual user actions (spam reports, deletions), and list quality. Sudden spikes in volume, even with high engagement, can trigger throttling as a precautionary measure.
Microsoft's traffic patterns: More sensitive to 'irregular mail traffic,' which includes uncharacteristic changes in volume or sending cadence. They tend to be more rigid and may require direct outreach to resolve throttling issues.
Actionable steps to mitigate throttling
The first step is vigilant monitoring. Regularly check your sender reputation dashboards like Google Postmaster Tools and Microsoft SNDS for any dips in reputation, increases in spam complaints, or warning signs. These tools, while not showing everything, are your primary indicators of potential issues. Also, keep a close eye on your bounce rates and blocklist (or blacklist) status.
If you've identified that throttling is occurring without obvious red flags, auditing your sending practices is crucial. Review your list acquisition methods to ensure only engaged subscribers are added. Implement a strict re-engagement strategy for inactive users and remove those who consistently don't engage. Breaking up large sends is a good practice, but ensure the new cadence doesn't appear abnormal to the ISPs.
When facing persistent throttling, especially from Microsoft, don't hesitate to reach out to their postmaster support teams. Be prepared with detailed sending data, including volume, engagement rates, complaint rates, and any changes in your sending infrastructure or practices. Providing concrete evidence can help them understand your situation and potentially lift the restrictions. For more on preventing deliverability issues, read our Email Deliverability Issues guide.
Throttling trigger
Solution/mitigation
Sudden volume changes
Implement gradual volume ramp-up (IP warming) even if moving segments. Maintain consistent cadence.
Hidden spam traps
Regularly clean lists, remove inactive users, use double opt-in.
Authentication issues
Verify SPF, DKIM, and DMARC records are correctly configured and aligned. Use DMARC monitoring for issues.
Excessive complaints
Segment based on engagement, provide easy unsubscribe options, improve content relevance. Check Postmaster Tools spam rate.
Views from the trenches
Best practices
Maintain a consistent sending volume and cadence to build a predictable sending profile with ISPs.
Segment your audience based on engagement levels to ensure you send to active and interested recipients.
Regularly clean your email lists to remove inactive addresses and potential spam traps.
Set up and monitor Google Postmaster Tools and Microsoft SNDS for real-time insights into your reputation.
Common pitfalls
Making sudden, significant changes to sending volume or frequency without gradually warming up.
Not actively monitoring complaint rates or bounce rates beyond basic reports.
Ignoring subtle warnings or 'yellow' statuses in ISP postmaster tools until they become 'red'.
Failing to engage directly with ISP postmaster teams when issues persist despite internal checks.
Expert tips
When contacting Microsoft support, provide extensive data and specific details about your consistent sending patterns.
Even minor changes in sending infrastructure can trigger new reputation evaluations by ISPs.
Focus on domain reputation as much as IP reputation, as domain reputation is increasingly critical for deliverability.
Consider how engagement metrics are calculated across different ISPs; they might interpret 'high engagement' differently.
Expert view
Expert from Email Geeks says they have noticed similar throttling issues with Microsoft, even with green statuses in SNDS.
2020-05-05 - Email Geeks
Expert view
Expert from Email Geeks says they received instant mitigation from Microsoft after pushing back with data, suggesting it was a false flag issue.
2020-05-05 - Email Geeks
Navigating modern email deliverability
Email throttling from major providers like Gmail and Microsoft, even with high engagement and good reputation, is a complex challenge. It often stems from a combination of subtle shifts in sending patterns, underlying list hygiene issues, or the evolving, stricter policies of the ISPs themselves. While your visible metrics might be excellent, the sophisticated algorithms of these providers detect anomalies that can trigger protective throttling measures.
By proactively monitoring your sender reputation across various platforms, meticulously auditing your list quality and sending practices, and being prepared to engage directly with postmaster teams with comprehensive data, you can navigate these challenges. Understanding these nuances and responding strategically is key to maintaining optimal email deliverability.
Gmail's dynamic throttling: Highly adaptive, responding to real-time engagement, individual user actions (spam reports, deletions), and list quality. Sudden spikes in volume, even with high engagement, can trigger throttling as a precautionary measure.
Microsoft's traffic patterns: More sensitive to 'irregular mail traffic,' which includes uncharacteristic changes in volume or sending cadence. They tend to be more rigid and may require direct outreach to resolve throttling issues.
